cmake: define BOOST_ASIO_USE_TS_EXECUTOR_AS_DEFAULT for Boost.Asio users

cmake: define BOOST_ASIO_USE_TS_EXECUTOR_AS_DEFAULT for Boost.Asio users

see also
https://www.boost.org/doc/libs/1_74_0/doc/html/boost_asio/std_executors.html#boost_asio.std_executors.polymorphic_i_o_executor

we could use `asio::any_io_executor` later on though for better
performance.

also, define CMP0093, so FindBoost reports Boost_VERSION in x.y.z
format. it is simpler to use `VERSION_GREATER_EQUAL` to compare its
version with 1.74 instead of its C macro version ("107000").

Signed-off-by: Kefu Chai <kchai@redhat.com>
(cherry picked from commit 3d708219092d0e89a1434c30ffc8a4999f062cc0)

Conflicts:
	CMakeLists.txt
	src/librbd/CMakeLists.txt: change ""Boost_VERSION
VERSION_GREATER_EQUAL "1.74" to "Boost_VERSION VERSION_GREATER
1.73" as "VERSION_GREATER_EQUAL" was introduced in a newer version of
CMake, which is not available in ubuntu/xenial.
	src/rgw/CMakeLists.txt: add the same macro definition applied to
          librbd. without which, src/rgw/rgw_sync_module_aws.cc
          still fails to compile with boost 1.74