Table: Do not attempt to prune an empty table

Table: Do not attempt to prune an empty table

Table pruning is requested from multiple places, including
a timer. Sometimes this may happen in a race condition with
table shutdown, and therefore we explicitly refuse to schedule
pruning if the table is empty.

Also added a full-blown check that a shutting-down table
is indeed empty, and several more asserts to catch imminent crashes
before they happen in hard-to-debug places.

This fixes #254.