disk space. added tagdb file cache for better performance,
less disk accesses. will help reduce disk load.
put file cache sizes in master controls and if they change
then update the cache size dynamically.
it's useful to limit per process mem usage to prevent
oom killer because we can't save if we get killed.
overhaul diskpagecache to just use rdbcache. much simpler
and faster, but disabled for now until debugged more.
reduce min files to merge for crawlbot collections so
they stay more tightly merged to conserve fds and mem.
improved logDebugDisk msgs.
overhauled File.cpp fd pool. now it is way faster and
doesn't use any extra mem. much simpler too. although
could be sped up a little by using a linked list, but
probably is not significant enough to warrant doing right now.
increase mem ptr table from 3M to 8M slots. should really make
dynamic though. fix core from null msg20s[0]->m_r.
only call attemptMergeAll once every 60 seconds really.
do not attempt merge if already merging.
