gc: run garbage collection in background

Currently, garbage collection is executed synchronously by functions
that may trigger it, such as gc_consumer_advance or gc_add_checkpoint.
As a result, one has to be very cautious when using those functions as
they may yield at their will. For example, we can't shoot off stale
consumers right in tx_prio handler - we have to use rather clumsy WAL
watcher interface instead. Besides, in future, when the garbage
collector state is persisted, we will need to call those functions from
on_commit trigger callback, where yielding is not normally allowed.

Actually, there's no reason to remove old files synchronously - we could
as well do it in the background. So this patch introduces a background
garbage collection fiber that executes gc_run when woken up. Now all
functions that might trigger garbage collection wake up this fiber
instead of executing gc_run directly.