The ledgernext and kvledger packages provide APIs to simulate
transactions by collecting read-write sets from invokes of
chaincodes. This change set integrates this for the Endorser
flows.  The main purpose of this code is to enable read write
sets to be propagated so end to end flow ending in a commit to
the ledger can be tested.

The chaincode unit tests continue to use the old ledger. This
allows us to (1) incrementally integrate ledger and (2) show
that the two packages can coexist from a build and runtime
point of view.

It is worth noting that the file kv_ledgers.go hosts a temporary
container for ledgers. This simple approach is expected to be
revised when (sub)ledgers are implemented.

Change-Id: I6e0bf4b9795b83d2ae869244b212c02ef9b5214a
Signed-off-by: Srinivasan Muralidharan <muralisr@us.ibm.com>