Significant decompression refactoring

This replaces a mishmash of pointers in the Block and BlockContents classes
with std::unique_ptr. It also changes the semantics of BlockContents to be
limited to use as a constructor parameter for Block objects, as it owns any
block buffers handed to it

Furthermore, it has a lot of refactoring. This was needed mostly within
ReadBlockContents, which did some tricky abuse of function calls and booleans
to switch between using a heap pointer or stack pointer. This drops that
boolean by replacing it with a simple pointer while at the same time allowing a
unique_ptr to handle any cleanup that might escape the previous version of
ReadBlockContents.

In many places, the placement of BlockContents and Status have been swapped to
take advantage of return value optimizations, automatically generated move
constructors, and to reduce the number of times a Status object is moved around
for an otherwise OK result.

Zero leaks have been detected since these changes, all unit tests pass, and
complexity is greatly reduced.

Reviews requested from @vinniefalco, @mtrippled, @nbougalis. None of them are owners of rocksdb (afaict) but considered relevant to Ripple's use of rocksdb.