INDY-1284: support read-only state in read_ledger with RocksDB.
INDY-1284: support read-only state in read_ledger with RocksDB.

Since read-only mode is supported for RocksDB a copy of
ledger is not created in case of RocksDB-based ledger.

Also added additional checks to avoid removing of the
ledger instead of it's copy.

Signed-off-by: Sergey Shilov <064bbdfaeb89a0bebebfe7b388747a73c8941704@dsr-company.com>

spivachuk/sovrin-node,spivachuk/sovrin-node,spivachuk/sovrin-node,spivachuk/sovrin-node