Improve FragmentTombstones() speed by lazily initializing seq_set_

[cbi42]

Summary: FragmentedRangeTombstoneList has a member variable seq_set_ that contains the sequence numbers of all range tombstones in a set. The set is constructed in FragmentTombstones() and is used only in FragmentedRangeTombstoneList::ContainsRange() which only happens during compaction. This PR moves the initialization of seq_set_ to FragmentedRangeTombstoneList::ContainsRange(). This should speed up FragmentTombstones() when the range tombstone list is used for read/scan requests, this is true for most cases when FragmentTombstones() is called for range tombstones of the mutable memtable. Microbench shows the speed improvement to be ~45%.

Test plan:

• Existing tests and stress test: python3 tools/db_crashtest.py whitebox --simple --verify_iterator_with_expected_state_one_in=5.
• Microbench: update range_del_aggregator_bench to benchmark speed of FragmentTombstones():

./range_del_aggregator_bench --num_range_tombstones=1000 --tombstone_start_upper_bound=50000000 --num_runs=10000 --tombstone_width_mean=200 --should_deletes_per_run=100 --use_compaction_range_del_aggregator=true

Before this PR:
=========================
Fragment Tombstones:     270.286 us
AddTombstones:           1.28933 us
ShouldDelete (first):    0.525528 us
ShouldDelete (rest):     0.0797519 us

After this PR: time to fragment tombstones is pushed to AddTombstones() which only happen during compaction.
=========================
Fragment Tombstones:     149.879 us
AddTombstones:           102.131 us
ShouldDelete (first):    0.565871 us
ShouldDelete (rest):     0.0729444 us

• db_bench: this should improve speed for fragmenting range tombstones for mutable memtable:

./db_bench --benchmarks=readwhilewriting --writes_per_range_tombstone=100 --max_write_buffer_number=100 --min_write_buffer_number_to_merge=100 --writes=500000 --reads=250000 --disable_auto_compactions --max_num_range_tombstones=100000 --finish_after_writes --write_buffer_size=1073741824 --threads=25

Before this PR: 
readwhilewriting :      18.301 micros/op 1310445 ops/sec 4.769 seconds 6250000 operations;   28.1 MB/s (41001 of 250000 found)
After this PR:
readwhilewriting :      16.943 micros/op 1439376 ops/sec 4.342 seconds 6250000 operations;   23.8 MB/s (28977 of 250000 found)

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[ajkr]

Nice finding, LGTM!

[cbi42]

Nice finding, LGTM!

Thanks for the review!