Similarly as with kvdb.View this commits adds a reset closure to the
kvdb.Update call in order to be able to reset external state if the
underlying db backend needs to retry the transaction.
This commit extends the etcd.BackendConfig to also provide an abort
context and integrates it with the STM retry loop in order to be able
stop LND when conflicting transactions keep the loop running.
This commit separates all etcd related sources (sans a few stubs and
config) from the rest of the source tree and makes compilation conditional
depending on whether the kvdb_etcd build tag is specified.
