Garbage Collection enhancements (prevent GC from locking repo)

#### Version information:
v0.4.19

#### Type:
enhancement

#### Description:
Currently running a garbage collection prevents the node from running operations like ipfs add while the garbage collection is running. In production environments this could become quite problematic as repos scale in size and garbage collection takes longer.

From conversations with @Stebalien, this is something that could possibly be improved by implementing local reference counts which should make GC trivial. But implementing that correctly is non-trivial (and may require an actual database).

If something like a database is implemented, it could also be utilized for metadata capabilities that have been requested for various different features.