Refactor FTDC

Changes the 'ftdc_mongod' library into two libraries:
ftdc_server - common code to be shared between mongod and mongos
ftdc_mongod - MongoD specific code

The refactor is just a copy paste with the following changes:
1. Add a RegisterCollectorsFunction so that mongod/mongos can register their own unique collectors.
2. Expose FTDCSimpleInternalCommandCollector as a collector in the header file that both MongoD and MongoS can use. This continues to remains separate from the 'ftdc' base library to ensure the the 'ftdc' library does not take a dependency on 'db/commands'. The implementation has not changed, only code motion.