* node: allow replacing existing p2p.Reactor(s)
using [`CustomReactors`
option](https://godoc.org/github.com/tendermint/tendermint/node#CustomReactors).
Warning: beware of accidental name clashes. Here is the list of existing
reactors: MEMPOOL, BLOCKCHAIN, CONSENSUS, EVIDENCE, PEX.
* check the absence of "CUSTOM" prefix
* merge 2 tests
* add doc.go to node package
