Update the FIFO and soft reference entry cache implementations to convert the
lock timeout attribute to use an integer with unit syntax rather than just an
integer.

Also, update the FIFO entry cache implementation to get rid of the calls to run
the garbage collector.  Instead, if we detect that too much memory is being
used, we'll not store the new entry and get rid of an old entry.  This is much
more palatable in terms of performance and response time when the cache gets
full.