reduce db mutex contention for write batch groups
Summary: This diff allows a Writer to join the next write batch group without acquiring any locks. Waiting is performed via a per-Writer mutex, so all of the non-leader writers never need to acquire the db mutex. It is now possible to join a write batch group after the leader has been chosen but before the batch has been constructed. This diff doesn't increase parallelism, but reduces synchronization overheads. For some CPU-bound workloads (no WAL, RAM-sized working set) this can substantially reduce contention on the db mutex in a multi-threaded environment. With T=8 N=500000 in a CPU-bound scenario (see the test plan) this is good for a 33% perf win. Not all scenarios see such a win, but none show a loss. This code is slightly faster even for the single-threaded case (about 2% for the CPU-bound scenario below). Test Plan: 1. unit tests 2. COMPILE_WITH_TSAN=1 make check 3. stress high-contention scenarios with db_bench -benchmarks=fillrandom -threads=$T -batch_size=1 -memtablerep=skip_list -value_size=0 --num=$N -level0_slowdown_writes_trigger=9999 -level0_stop_writes_trigger=9999 -disable_auto_compactions --max_write_buffer_number=8 -max_background_flushes=8 --disable_wal --write_buffer_size=160000000 Reviewers: sdong, igor, rven, ljin, yhchiang Subscribers: dhruba Differential Revision: https://reviews.facebook.net/D43887
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