Add support for multiple workers for mutations

Mutation Manager now has multiple workers for handle incoming mutation
queue. A single vbucket cannot be serviced by multiple goroutines
as sequences numbers needs to be persisted in sequence always.

The new approach has a configurable number of channels which handle
a set of vbuckets. A single goroutine will service a channel.
Set MaxOpenFiles for leveldb to 500

LevelDB can consume lot of file handles. Need a upper limit.
This limit is per index.