Multi-Database Support

[TalZaccai]

This PR enables multiple logical databases in a single Garnet server instance.

This PR includes:

• Adding MaxDatabases configuration to garnet.conf + compatibility with redis.conf
• Multi-DB support in StoreWrapper, including checkpointing & AOF
• Current-DB context switching support in RespServerSession
• Update GarnetInfoMetrics to reflect multiple databases data
• Support for the following commands:
  • SELECT - switch current DB context
  • SWAPDB - swap two DB instances
  • FLUSHDB - flush all keys from current DB in context
  • FLUSHALL - flush all keys from all active databases
  • RANDOMKEY - get a random key from the current DB in context
• Add support for specifying a DB ID in the following commands:
  • SAVE - synchronous checkpointing database by ID
  • BGSAVE - asynchronous checkpointing database by ID
  • LASTSAVE - number of seconds since last checkpoint of database by ID
  • COMMITAOF - synchronous database AOF commit by ID
• Add DB ID to client info
• Testing for supported commands
• Benchmarking for supported commands