BUG-650: remove executor abstraction

This patch removes sameThreadExecutor from the commit path, eliminating
associated overhead. Relevant benchmarks show improvement pretty much
across the board:

BEFORE                                                            millis        error
write100KSingleNodeWithOneInnerItemInCommitPerWriteBenchmark    2213.735       77.597
write100KSingleNodeWithOneInnerItemInOneCommitBenchmark          171.524        2.289
write10KSingleNodeWithTenInnerItemsInCommitPerWriteBenchmark     164.282        1.391
write10KSingleNodeWithTenInnerItemsInOneCommitBenchmark           14.161        0.196
write50KSingleNodeWithTwoInnerItemsInCommitPerWriteBenchmark     982.697       29.397
write50KSingleNodeWithTwoInnerItemsInOneCommitBenchmark           93.233        2.174

AFTER                                                             millis        error   delta
write100KSingleNodeWithOneInnerItemInCommitPerWriteBenchmark    2138.900       75.844   -3.4%
write100KSingleNodeWithOneInnerItemInOneCommitBenchmark          177.839        3.997   +3.5%
write10KSingleNodeWithTenInnerItemsInCommitPerWriteBenchmark     158.666        1.090   -3.5%
write10KSingleNodeWithTenInnerItemsInOneCommitBenchmark           13.022        0.105   -8.0%
write50KSingleNodeWithTwoInnerItemsInCommitPerWriteBenchmark     935.490       30.395   -4.8%
write50KSingleNodeWithTwoInnerItemsInOneCommitBenchmark           89.907        1.204   -3.6%

Furthermore it cleans up and marks FIXMEs for defunct statistics. These
will need to be replaced with implementation which does not assume
underlying implementation.

Change-Id: I01c51462a8529a2f874ecd2f9af05faba503bc58
Signed-off-by: Robert Varga <rovarga@cisco.com>