Bug 1073: Implemented Transaction chain on InMemoryDOMDataStore level.

- Extracted Transactions implementations from InMemoryDOMDataStore
to separate classes
- implemented initial DOMStoreTransactionChain inside InMemoryDOMDataStore

Change-Id: I3033f26d2a4c141a32aaac6bc3a20e8c7ba1c28c
Signed-off-by: Tony Tkacik <ttkacik@cisco.com>
Merge "Bug 1073: Implemented Transaction chain on InMemoryDOMDataStore level."