vinyl: rotate metadata log on checkpoint

 - Name vinyl metadata log as <lsn>.vymeta, where <lsn> is the last
   snapshot signature.
 - Create a new log on checkpoint, discarding records left from
   dead indexes and records that cancel each other.
 - Log rotation is done asynchronously from a coeio thread so
   as not to stall ongoing transactions.
 - While the log is being rotated, all log writers are locked out.
   This should be OK, because log is rotated fairly quickly and
   the scheduler, which is the primary user of the log, runs rarely.

Closes #2031