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.

Finally, perf timers have been refactored to automatically stop (exactly the same as calling ```PERF_TIMER_STOP(metric))``` by way of handling this in their destructors. Zero significant change in performance metrics has been observed. This eliminates the burden of developers remembering to call ```PERF_TIMER_STOP(metric)``` immediately before any return statement which brings more correctness and predictability into their use.

Zero leaks have been detected since these changes, all unit tests pass, and complexity is greatly reduced.
Can you explain why we still leave data_, size_, cachable_ and compression_type_ in class Block? It seems to me that in the case of not owned, we should fill the respective variables in contents_ and remove data_, size_, restart_offset_, cachable_ and compression_type_ from class Block.