WIP: file-based back end for bake
Do not merge; this is a prototype for preliminary performance testing.
The point of this experiment is to have the ability to store regions in normal POSIX files using O_DIRECT as an alternative to storing regions in libpmemobj objects.
This branch does not implement reads, removes, eager operations, or size checks. It is just enough for write performance benchmarking.
All regions are stored in a single log-structured files, with pwrite() and fdatasync() calls serviced via an abt-io instance backed by 8 execution streams.
Required: run with pipelining enabled. Recommended: run with no dedicated threads for RPC handling.