rgw/dbstore: free objectmap in DB::Destroy()

rgw/dbstore: free objectmap in DB::Destroy()

Memory leak was detected by AddressSanitizer in dbstore tests. Objects
inserted into a static objectmap were not being properly freed when
tests completed without explicitly deleting buckets.

The leak occurred because:

- Objects were preserved in a static map after insertion
- Objects were only freed by DB::objectmapDelete() when deleting the
  corresponding bucket
- In tests, buckets weren't always deleted after insertion, leaving
  objects allocated

ASan rightly pointed this out:

```
Direct leak of 200 byte(s) in 1 object(s) allocated from:
    #0 0x55c420f5274d in operator new(unsigned long) (/home/jenkins-build/build/workspace/ceph-pull-requests/build/bin/unittest_dbstore_tests+0x4df74d) (BuildId: c19ed2d2b1ead306fdc59fc311f546a287300010)
    #1 0x55c42143cfaf in SQLGetBucket::Execute(DoutPrefixProvider const*, rgw::store::DBOpParams*) /home/jenkins-build/build/workspace/ceph-pull-requests/src/rgw/driver/dbstore/sqlite/sqliteDB.cc:1689:11
    #2 0x55c42102751c in rgw::store::DB::ProcessOp(DoutPrefixProvider const*, std::basic_string_view<char, std::char_traits<char>>, rgw::store::DBOpParams*) /home/jenkins-build/build/workspace/ceph-pull-requests/src/rgw/driver/dbstore/common/dbstore.cc:251:16
    #3 0x55c4210328c9 in rgw::store::DB::get_bucket_info(DoutPrefixProvider const*, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>> const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>> const&, RGWBucketInfo&, std::map<std::__cxx11
::basic_string<char, std::char_traits<char>, std::allocator<char>>, ceph::buffer::v15_2_0::list, std::less<std::__cxx11::basic_string<char, std::c
har_traits<char>, std::allocator<char>>>, std::allocator<std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>> const, ceph::buffer::v15_2_0::list>>>*, std::chrono::time_point<ceph::real_clock, std::chrono::duration<unsigned long, std::ratio<1l, 1000000000l>>>*, obj_version*) /home/jenkins-build/build/workspace/ceph-pull-requests/src/rgw/driver/dbstore/common/dbstore.cc:450:9
    #4 0x55c421034357 in rgw::store::DB::create_bucket(DoutPrefixProvider const*, std::variant<rgw_user, rgw_account_id> const&, rgw_bucket const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>> const&, rgw_placement_rule const&, std::map<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>>, ceph::buffer::v15_2_0::list, std::less<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>>>, std::allocator<std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>> const, ceph::buffer::v15_2_0::list>>> const&, std::optional<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>>> const&, std::optional<RGWQuotaInfo> const&, std::optional<std::chrono::time_point<ceph::real_clock, std::chrono::duration<unsigned long, std::ratio<1l, 1000000000l>>>>, obj_version*, RGWBucketInfo&, optional_yield) /home/jenkins-build/build/workspace/ceph-pull-requests/src/rgw/driver/dbstore/common/dbstore.cc:504:9
    #5 0x55c420f6c3ed in DBStoreTest_CreateBucket_Test::TestBody() /home/jenkins-build/build/workspace/ceph-pull-requests/src/rgw/driver/dbstore/tests/dbstore_tests.cc:495:13
    #6 0x55c42174fa0d in void testing::internal::HandleSehExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*) /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:2653:10
    #7 0x55c421717f25 in void testing::internal::HandleExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*) /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:2689:14
    #8 0x55c4216d4bbd in testing::Test::Run() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:2728:5
```

In this change, we:

- Free objectmap entries in DB::Destroy() to ensure cleanup on shutdown
- Call DB::Destroy() in DBStoreTest::TearDown() to guarantee cleanup after each test

Signed-off-by: Kefu Chai <tchaikov@gmail.com>