bdr: Replace GUC-based connections with SQL and tables

bdr: Replace GUC-based connections with SQL and tables

BDR's configuration interface and the related internals have been mostly
rewritten so that GUCs are not used for specifying bdr connections.

The bdr.connectons GUC and bdr.<nodename>_dsn etc are removed by this
commit. Use of these settings will now emit warnings in the server error
log.

Upgrades to the new version must be performed with manual steps covered
in the documentation. Simply replacing the binaries is not sufficient.

Major changes:

* There is only one static worker, a new bdr_supervisor worker

* The supervisor decides which databases to start per-db workers
  for by examining each database for a 'bdr' security label

* per-db workers scan a new bdr.bdr_connections table to decide
  which nodes to launch apply workers for.

* apply workers look up bdr.bdr_connections to get connection
  info instead of checking GUCs

* Explicit creation of the first node is now required with
  bdr.bdr_group_create(...)

* New nodes must be joined with an SQL level bdr.bdr_group_join(...)
  call.