Feature request: allow multiple injectors and mergers

[matthewdarwin]

In the current dfuse architecture only one merger, trxdb loader and statedb loader is allowed (vs other components where you can run apps concurrently)

It would be nice if multiple of each could be running concurrently and they use dmesh to make one primary (active) and the other ones standby. If the active one fails, then one of the standby ones takes over active.