Bug 1073: Added Transaction Chain support to InMemoryDataTreeModification.

Added support for chaining transactions to underlying implementation.

Added test case testTransactionChain, which is documented
and illustrates basic behaviour of transaction chain -
local virtual memory, which allows to allocate new transactions
which provides view as-if previous transaction already happened.

Change-Id: I23f5622f9a6498356b3c54d53e0111f3fba33bf5
Signed-off-by: Tony Tkacik <ttkacik@cisco.com>
Merge "Bug 1073: Added Transaction Chain support to InMemoryDataTreeModification."