Summary:
There's no need for WriteImpl to flatten the write batch group
into a single WriteBatch if the WAL is disabled.  This diff moves the
flattening into the WAL step, and skips flattening entirely if it isn't
needed.  It's good for about 5% speedup on a multi-threaded workload
with no WAL.

This diff also adds clarifying comments about the chance for partial
failure of WriteBatchInternal::InsertInto, and always sets bg_error_ if
the memtable state diverges from the logged state or if a WriteBatch
succeeds only partially.

Benchmark for speedup:
  db_bench -benchmarks=fillrandom -threads=16 -batch_size=1 -memtablerep=skip_list -value_size=0 --num=200000 -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

Test Plan: asserts + make check

Reviewers: sdong, igor

Reviewed By: igor

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D50583