New class Utils::SessionKeepAlive: for keeping sessions alive

[mwarin]

Broke out some code from lib/overlap/overlap_table_update.rb into new class lib/utils/session_keep_alive.rb, since the same thing was needed to keep the holdings deleter from crashing with an expired cursor.

@jsteverman and I tried to think what a (useful && non-production) test for this would look like. overlap_table_update.rb does not directly test this behavior.

Any ideas from @aelkiss or the Universe appreciated.

[aelkiss]

After looking a bit, I'd suggest:

• put this in a separate tagged test we don't run by default (because it will be slow)
• set the mongodb session timeout to 1 minute: https://docs.mongodb.com/manual/reference/command/startSession/ -- AFAICT the timeout can't be set below that
• without timeout avoidance set, run an artificially slow mongodb query (https://stackoverflow.com/questions/12908425/simulate-slow-query-in-mongodb) and verify that you get the missing cursor error: run an artificially slow mongodb query:
• with timeout avoidance set, run the same query and verify that it returns successfully

[jsteverman]

Looks good.