Implements NEM (XOS) maintenance mode

XOS should be extended to support a maintenance mode. Proposed mode options include the following:

• NORMAL - operate normally
• DRAIN - operations currently in progress may be completed, but new operations should not be started.
• READONLY - XOS can be queried for information, but writes will be denied. This is more restrictive than DRAIN mode, as operations in progress will fail as they cannot be written.
• OFFLINE - XOS supports neither read nor write of information. (Is this useful?)

One use case for these modes is external backup of XOS's postgres database. The system could be put into DRAIN mode, once everything has been drained be put into READONLY mode, then an operator could issue a postgress dump of the database knowing that the database was static during the dump.