Move Blockstorage code under /fabric/common package

https://jira.hyperledger.org/browse/FAB-2022



This changes introduced by this CR
- Moves the block storage code from package
core/ledger/blkstorage to common/ledger/blkstorage

- Splits the ledger_interface.go so as to move common interfaces
and data type to common/ledger package

- Moves some of the util functions to common/ledger package

- Moves core/ledger/ordererledger package to orderer/ledger/fsledger
orderer folks can futher rename/refactor this as seems suitable to them

Change-Id: I759e16f00dc2ec9bb62196121083cf48eae76948
Signed-off-by: manish <manish.sethi@gmail.com>