https://jira.hyperledger.org/browse/FAB-602

The refactoring allows sharing of transaction execution and validation code across goleveldb and couchdb.
This refactoring also allows plugging-in different schemes for various aspects of transaction execution
such as maintianing isolation during simulation, validation and commit.

Change-Id: I0d3fb1cdff99c3f6f991e7b7469920173f194a87
Signed-off-by: manish <manish.sethi@gmail.com>