Remove copying of range tombstones keys in iterator (#10878)

Remove copying of range tombstones keys in iterator (#10878)

Summary:
In MergingIterator, if a range tombstone's start or end key is added to minHeap/maxHeap, the key is copied. This PR removes the copying of range tombstone keys by adding InternalKey comparator that compares `Slice` for internal key and `ParsedInternalKey` directly.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10878

Test Plan:
- existing UT
- ran all flavors of stress test through sandcastle
- benchmarks: I did not get improvement when compiling with DEBUG_LEVEL=0, and saw many noise. With `OPTIMIZE_LEVEL="-O3" USE_LTO=1` I do see improvement.
```
# Favorable set up: half of the writes are DeleteRange.
TEST_TMPDIR=/tmp/rocksdb-rangedel-test-all-tombstone ./db_bench --benchmarks=fillseq,levelstats --writes_per_range_tombstone=1 --max_num_range_tombstones=1000000 --range_tombstone_width=2 --num=1000000 --max_bytes_for_level_base=4194304 --disable_auto_compactions --write_buffer_size=33554432 --key_size=50

# benchmark command
TEST_TMPDIR=/tmp/rocksdb-rangedel-test-all-tombstone ./db_bench --benchmarks=readseq[-W1][-X5],levelstats --use_existing_db=true --cache_size=3221225472  --disable_auto_compactions=true --avoid_flush_during_recovery=true --seek_nexts=100 --reads=1000000 --num=1000000 --threads=25

# main
readseq [AVG    5 runs] : 26017977 (± 371077) ops/sec; 3721.9 (± 53.1) MB/sec
readseq [MEDIAN 5 runs] : 26096905 ops/sec; 3733.2 MB/sec

# this PR
readseq [AVG    5 runs] : 27481724 (± 568758) ops/sec; 3931.3 (± 81.4) MB/sec
readseq [MEDIAN 5 runs] : 27323957 ops/sec; 3908.7 MB/sec
```

Reviewed By: ajkr

Differential Revision: D40711170

Pulled By: cbi42

fbshipit-source-id: 708cb584e2bd085a9ce0d2ef6a420489f721717f