Fixed vacuum/snapshot performance issues

Vacuum is now ran in parallel with disk sync and retrieves only records
that need to be vacuumed by iid, without full scan of the state using
linked list. Now works faster and more reliable.

Snapshot writing has been desynchronized using random delay, since it
has a performance hit when ran synchronously on all nodes.
Added bytes written to stats logging
Fixed mutex lock/unlock asymmetry

It lead to unlock for a mutex not owned by current fiber, which
resulted in assert failure later in libevfibers.