MongoDB compound index sharding support

Features:

• MongoDB limiter supports compound unique index sharding with indexKeyPrefix option now.
• disable prefixing keys with empty keyPrefix option.
• mongodb: specify collection name with tableName option.

Bug fixes:

• postgresql: fix bug storing limits on different tables.
• implement toString, symbol inspect and toJSON for RateLimiterRes.
• fix RateLimiterUnion's consume TS response type (thanks @zephraph )
• add type signatures for toString and toJSON methods of RateLimiterRes (thanks @michaeldzjap )