Support bootstrapping spilo cluster from the external PostgreSQL database

[zalando]

Support bootstrapping spilo cluster from the external PostgreSQL database

[alexeyklyukin]

We might still need it in some form or another. Let's keep the ticket open for now.

[CyberDem0n]

I believe #252 solves this issue

[alexeyklyukin]

The particular plan did not work out (*): after playing with the standby mode flag we've decided the interactions with the standby mode flag and the decision on when to clear this flag (given that the "primary standby" node may just crash or shut down) were too complex to manage them automatically, so instead, we would rely on a client to set the keys in DCS in order for the existing cluster to point elsewhere, and clear those keys when we are done. The user setting up the external master would also be responsible for creating replication slots or setting up WAL archiving.

Of course, we need the client support and documented process on how to set things up, which will be a part of this task.

• https://github.com/zalando/patroni/tree/feature/copy_existing_cluster

[LappleApple]

@alexeyklyukin Hi! Any movement on this since Oct 2015? LMK if we can close.

[LappleApple]

@alexeyklyukin Given @CyberDem0n's last comment, can we close now?