Add restartability support in mutation manager

Expose GetSequenceVector RPC method which provides the SequenceVector for each index
Persist SequenceVector to BackIndex as a MetaDoc for every mutation
Retrieve SequenceVector from storage at init time for exisiting indexes
Restartability support for indexer

- Do not persist engine as part of catalog
- Engine is stored in a map at runtime now
- When indexer starts, it will open the engine for existing indexes
- Same map is sent to mutation manager for initialization
Sample code for using the new GetSequenceVector

Sample code to demonstrate how to use the new GetSequenceVector RPC method
New methods for restartability support

InsertMeta/GetMeta methods to Persister Interface to allow persistence of SequenceVector
Related datastructures to support handling of SequenceVector
Change vbucket/seqno to uint64 from int64
Change vbucket/seqno from int64 to uint64
Implementation for InsertMeta/GetMeta methods

These methods allow SequenceVector to be persisted per index