Partial implementation for write & remove index entries.

Filled out stubs at client and server side except for IndexletManager.
Lint and minor bug fixes.
IndexedRead: Implemented client side, improved unit test, fixed bugs,
improved semantics of rpc and documentation.
IndexedRead: Server side improvement + bug fixes + unit test.

Now there is better division of labor between various classes
involved such that there is minimal information leakage.
Indexing Schema Management RPCs:
1) CreateIndex
2) DropIndex
3) TakeIndexletOwnership
4) DropIndexletOwnership

IndexletManager handles schema related changes for indexlets on master servers.
Currently assuming "string" type while creating new index partitions and for
comparison in IndexletManager.