replace validator in Tendermint node from `init-validator` tx

[tzemanovic]

Currently, after creating a new validator account on-chain with the init-validator tx, it is necessary to manually restart the ledger node for Tendermint to use the new validator key.

We could allow the node to automatically pick up a newly created validator with init-validator, write it to Tendermint's dir and restart the Tendermint process. We should be able to re-use whatever approach we use for consensus key change in https://github.com/anoma/namada/issues/36.

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[juped]

can tendermint be asked to reload without restarting as well, perhaps with SIGHUP or some request over the ipc?

[cwgoes]

@brentstone can you check / take this on after the PoS stuff since it is related?

[brentstone]

@brentstone can you check / take this on after the PoS stuff since it is related?

Yup.

[cwgoes]

Not important.