etcd: STM to allow prefetching of keys and full ranges

This commit adds more full range caching to the STM such that we'll
be able to prefetch keys and ranges on the kvdb level. This will allow
us to reduce roundtrips to etcd do fast iteration of small ranges.

kvdb/go.mod

@@ -3,9 +3,11 @@ module github.com/lightningnetwork/lnd/kvdb
 require (
 	github.com/btcsuite/btclog v0.0.0-20170628155309-84c8d2346e9f
 	github.com/btcsuite/btcwallet/walletdb v1.3.6-0.20210803004036-eebed51155ec
+	github.com/google/btree v1.0.1
 	github.com/lightningnetwork/lnd/healthcheck v1.0.0
 	github.com/stretchr/testify v1.7.0
 	go.etcd.io/bbolt v1.3.6
+	go.etcd.io/etcd/api/v3 v3.5.0
 	go.etcd.io/etcd/client/pkg/v3 v3.5.0
 	go.etcd.io/etcd/client/v3 v3.5.0
 	go.etcd.io/etcd/server/v3 v3.5.0