Reuse DB connections in Main thread

[timvaillancourt]

In version 0.2.0 we do not always reuse Mongo DB connections. DB connections cannot be reused across threads (must be reopened in each thread), but there are many connections in the "MainProcess" thread that could be reused by passing a DB class from Backup.py to ReplsetHandler.py + ShardingHandler.py.

Here is the DB:connect debug output for a 2-shard cluster (11 x DB connections created!!!):

[tim@centos7 mongodb_consistent_backup]$ grep 'DB:connect' log 
[2016-07-08 19:41:40,493] [DEBUG] [MainProcess] [DB:connect:23] Getting MongoDB connection to centos7-mongos1:27017
[2016-07-08 19:41:40,500] [DEBUG] [MainProcess] [DB:connect:23] Getting MongoDB connection to centos7-mongos1:27017
[2016-07-08 19:41:50,602] [DEBUG] [MainProcess] [DB:connect:23] Getting MongoDB connection to centos7-mongos1:27017
[2016-07-08 19:41:50,612] [DEBUG] [MainProcess] [DB:connect:23] Getting MongoDB connection to centos7-mongod1:27017
[2016-07-08 19:41:50,626] [DEBUG] [MainProcess] [DB:connect:23] Getting MongoDB connection to centos7-mongod3:27017
[2016-07-08 19:41:50,836] [DEBUG] [OplogTail-1] [DB:connect:23] Getting MongoDB connection to centos7-mongod2:27017
[2016-07-08 19:41:50,839] [DEBUG] [OplogTail-2] [DB:connect:23] Getting MongoDB connection to centos7-mongod4:27017
[2016-07-08 19:41:50,856] [DEBUG] [MainProcess] [DB:connect:23] Getting MongoDB connection to centos7-mongos1:27017
[2016-07-08 19:41:50,884] [DEBUG] [MainProcess] [DB:connect:23] Getting MongoDB connection to centos7-mongos1:27017
[2016-07-08 19:41:50,890] [DEBUG] [MainProcess] [DB:connect:23] Getting MongoDB connection to centos7-mongod1:27017
[2016-07-08 19:41:50,905] [DEBUG] [MainProcess] [DB:connect:23] Getting MongoDB connection to centos7-mongod3:27017