Support for HA deployment

Many users run ODS in a HA setup. With the current implementation the mechanism for doing this has some restrictions. This issue is opened to investigate what uses cases for HA deployment ODS should support and then to track any development work needed to implement them.

Comments from users to consider in this work include:

• The main use case is a master and a backup signer where easy failover is required should the master fail.
• A further use case to consider is 2 independent ODS signers, both with their own HSMs with the same pre-generated keys and the same config files. Requirement would be that when the signers are started they should run independently and in parallel and choose the same keys for signing.
• Also in the current enforcer the signing is determined by an interval, it does not occur at an absolute time and so slight differences between the times the enforcer runs on the two signers can lead to different keys being chosen.
• There is also a use case where a new, unsigned zone must be signed and published in a very short timescale (minutes) and therefore the enforcer must be constantly available.
• The solution should be robust with regard to SA's accidentally starting multiple enforcers