Add to registrar.go method UpdateChannel similar to JoinChannel, but … #771

Job	Run time
Basic Checks	3m 19s
Unit Tests	8m 3s
Integration Tests (raft)	12m 22s
Integration Tests (smartbft)	20m 12s
Integration Tests (ledger)	12m 15s
Integration Tests (pvtdata)	11m 21s
Integration Tests (gateway idemix pkcs11 configtx configtxlator)	16m 59s
Integration Tests (lifecycle)	6m 49s
Integration Tests (discovery gossip devmode pluggable)	7m 52s
Integration Tests (pvtdatapurge e2e)	19m 37s
Integration Tests (sbe nwo msp)	8m 2s
	2h 6m 51s

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