Every level can have overlapping files.

Every level can have overlapping files.

Summary:
Leveldb has high write amplification because one file from level n
is compacted with all overlapping files in level n+1. This method
of compaction reduces read amplification (becasue there is only
one file to inspect per level) but the write amplification is high.

Another option would be to compact multiple files from the same
level and push it to a new file in level n+1. This means that
there will be overlapping files in each level. Each read
request might have to inspect multiple files at each
level. This could increase read amplification but should reduce
write amplification. This is called the "Hybrid" mode of
operations.

This patch introduces the "Hybrid" mode of operations (this deserves
a better name?). In the Hybrid mode, all levels can have overlapping
files. The number of files in a level determine whether compaction
is needed or not. Files in higher levels are larger in size that
files in lower levels. The default option is to have files of size
10MB in L1, 100MB in L2, 1000MB in L3, and so on.  If the number of
files in any level exceed 10 files, then that level is a target
for compactions. A compaction process takes many files from
level n and produces a single file in level n+1. The number of
files that are picked as part of a single compaction run is
limited by the size of the output file to be produced at the next
higher level.

This patch was produced by a two-full-day Christmas-Hack of 2012.

Test Plan:
All unit tests pass.

This patch switches on the Hybrid mode by default. This is done so
that all unit tests pass with the Hybrid Mode turned on. At time of
commit, I will switch off the Hybrid Mode.

Differential Revision: https://reviews.facebook.net/D7647