Suggested improvement to journal file: flush more often

to avoid pending writes.

In the event of process termination the journal file
is force-closed. Any journal writes that are buffered but
not yet flushed to disk will be missing. There are a couple
of places where a write to the journal file is not flushed
immediately to disk.

One of the cases (writeUtf8(REMOVE, ...) is called in a loop
as part of trimToSize(), which could lead to a substantial
number of deletes having taken place without
being flushed to the journal. I assume
the write would be flushed eventually when the 8k buffer is
filled (?) or until another write takes place that *is*
flushed.

This change may make cache read and deletion slower but lead to
reduced journal truncation in the event of a failure.

Both the cases are open for debate:

I can see the REMOVE case being flushed after the trimToSize()
rather than after each individual removal. Flushing after the
trimToSize() would still be an improvement, IMO.

I could see the READ case not needing to be flushed at all in the
name of performance. Not flushing in this case would, presumably,
just lead to out-of-date LRU information on restart.