pop3: reduce memory use while generating the mailbox cache

While the cache itself is relatively compact for 50K messages,
generating it was inefficient due to our schema and Over.pm APIs
being designed for NNTP.  While we won't change our schema for
now, we can choose better DBI APIs to use and limit our ephemeral
memory use.

This amounts to a 60% reduction in memory usage and a 5-10%
speedup against org.kernel.vger.git.0:

	{
		echo 'USER '$(uuidgen)'@org.kernel.vger.git.0'
		echo PASS anonymous
		echo STAT
		echo QUIT
	} | nc $HOST $PORT

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