Add the ability to cache most recent frame in memory

[khuck]

Currently, analysis scripts that are live-streaming need to query the DB to get the most recent frame. The most recent frame(s) of data (for all tbldata values) needs to be stored in a cache.

• the number of frames should be configurable

[cdwdirect]

Done. Reminder that attaching to an existing database does not retroactively populate the cache, only newly published values will be retained in the cache.

//Configure daemon to store 10 frames for this pub.
SOS_pub_config(pub, SOS_PUB_OPTION_CACHE, 10);

//Query the local SOS cache for all pub titles, all value names, latest frame, one deep:
SOSA_cache_grab(SOS, "", "", -1 1, "localhost", 22500); //Query all pubs & values for latest single frame.

Default cache depth is set in sos_options.c Cache is always enabled, but defaults to zero depth.