Begin moving WAL-specific I/O into the VFS.  This checkin contains VFS
infrastructure but it is untested and is not yet hooked up to the WAL.
The version number is bumped to 3.7.0 because of the VFS extension.

FossilOrigin-Name: f5e615c28c7035a7e6d896790b51cf9bc7371d5f
13 files changed