Periodic garbage collection can take a long time on large folders. The worst
step is the one for blocks, which are typically orders of magnitude more
numerous than files or block lists.
This improves the situation in by running blocks GC in a number of smaller
range chunks, in random order, and stopping after a time limit. At most ten
minutes per run will be spent garbage collecting blocklists and blocks.
With this, we're not guaranteed to complete a full GC on every run, but
we'll make some progress and get there eventually.
Signed-off-by: Jakob Borg <jakob@kastelo.net>
