Refactored server into index_manager and indexer

server.go is divided into index_manager/index_manager.go
and indexer/indexer.go

Indexer is a separate process which receives Create/Drop
from Index Manager and Scan/Stats from Index Client (query side)

Original indexer.go is now catalog.go(as it implements
disk catalog for index manager). Catalog is now a disk
based catalog without the responsibility for generating index uuid.
Index Manager is doing that part.

Unit tests have been updated as well
gofmt the code
Changes to IndexCatalog and IndexInfo

Add Exists method to IndexCatalog
Renamed Index to Engine for the Finder field
Change NewIndex to NewIndexEngine