node: allow replacing existing p2p.Reactor(s) (#3846)

* 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

node/doc.go

@@ -0,0 +1,40 @@
+/*
+Package node is the main entry point, where the Node struct, which
+represents a full node, is defined.
+
+Adding new p2p.Reactor(s)
+
+To add a new p2p.Reactor, use the CustomReactors option:
+
+		node, err := NewNode(
+				config,
+				privVal,
+				nodeKey,
+				clientCreator,
+				genesisDocProvider,
+				dbProvider,
+				metricsProvider,
+				logger,
+				CustomReactors(map[string]p2p.Reactor{"CUSTOM": customReactor}),
+		)
+
+Replacing existing p2p.Reactor(s)
+
+To replace the built-in p2p.Reactor, use the CustomReactors option:
+
+		node, err := NewNode(
+				config,
+				privVal,
+				nodeKey,
+				clientCreator,
+				genesisDocProvider,
+				dbProvider,
+				metricsProvider,
+				logger,
+				CustomReactors(map[string]p2p.Reactor{"BLOCKCHAIN": customBlockchainReactor}),
+		)
+
+The list of existing reactors can be found in CustomReactors documentation.
+
+*/
+package node