https://jira.hyperledger.org/browse/FAB-2810



This CR:
- Introduces proto messages
- Removes custom types for read-write set in package kvledger/txmgmt/rwset
- Renames the package kvledger/txmgmt/rwset to kvledger/txmgmt/rwsetutil
  and rwset.go to rwset_builder_test.go in order to reflect the purpose
  of the remaining code better
- Modifies the other packages to use rwset from protos
  and utilities from rwsetutil package

Change-Id: Id01be87903ad30f0eea34ef4033195b17bec3e71
Signed-off-by: manish <manish.sethi@gmail.com>