Serialization the get_mset steps due the concurrency produce a segmentation fault.

Fixed

• Segmentation fault in get_mset
• Added asserts on humanize inputs
• Added refs() funtion on shards
• Added assert on checkin for refs()
• Setting empty db on enquire inside get_mset for down db refs counter
• If prepared_mset always setting empty db in Enquire::Internal::set_database
• Fix re-use of readable databases
• Prevent locking forever, check pending every now and then