e2e: allow running of single node using the e2e app

[cmwaters]

This PR allows for the e2e app to be used outside of the e2e harness. This could be useful for larger, geographically-dispersed, and long-living testnets.

Running the e2e app comes with the following options:

Built-in

make node
tendermint init validator
TMHOME=$HOME/.tendermint ./build/node ./node/built-in.toml

To make things simpler the e2e application can also be run in the tendermint binary by running

tendermint start --proxy-app test

Socket

make node
tendermint init validator
tendermint start
./build/node ./node.socket.toml

Part of my reasoning for this change is to have the e2e app potentially replace the persistent kvstore (and perhaps kvstore).

[creachadair]

I like this idea. I have a few suggestions, but no real concerns.