vinyl: implement rebootstrap support

If vy_log_bootstrap() finds a vylog file in the vinyl directory, it
assumes it has to be rebootstrapped and calls vy_log_rebootstrap().
The latter scans the old vylog file to find the max vinyl object id,
from which it will start numbering objects created during rebootstrap to
avoid conflicts with old objects, then it writes VY_LOG_REBOOTSTRAP
record to the old vylog to denote the beginning of a rebootstrap
section. After that initial join proceeds as usual, writing information
about new objects to the old vylog file after VY_LOG_REBOOTSTRAP marker.
Upon successful rebootstrap completion, checkpoint, which is always
called right after bootstrap, rotates the old vylog and marks all
objects created before the VY_LOG_REBOOTSTRAP marker as dropped in the
new vylog. The old objects will be purged by the garbage collector as
usual.

In case rebootstrap fails and checkpoint never happens, local recovery
writes VY_LOG_ABORT_REBOOTSTRAP record to the vylog. This marker
indicates that the rebootstrap attempt failed and all objects created
during rebootstrap should be discarded. They will be purged by the
garbage collector on checkpoint. Thus even if rebootstrap fails, it is
possible to recover the database to the state that existed right before
a failed rebootstrap attempt.

Closes #461