multi: add sqlite backend option

In this diff, we expose the option to use sqlite as a db backend.

sample-lnd.conf

@@ -1199,7 +1199,8 @@ litecoin.node=ltcd
 [db]
 
 ; The selected database backend. The current default backend is "bolt". lnd
-; also has experimental support for etcd, a replicated backend.
+; also has experimental support for etcd, a replicated backend, postgres and
+; sqlite.
 ; db.backend=bolt
 
 ; The maximum interval the graph database will wait between attempting to flush
@@ -1275,6 +1276,22 @@ litecoin.node=ltcd
 ; Otherwise errors may occur in lnd under high-load conditions.
 ; db.postgres.maxconnections=
 
+[sqlite]
+; Sqlite connection timeout. Valid time units are {s, m, h}. Set to zero to
+; disable.
+; db.sqlite.timeout=0s
+
+; Maximum number of connections to the sqlite db. Set to zero for unlimited.
+; db.sqlite.maxconnections=0
+
+; The maximum amount of time to wait to execute a query if the db is locked.
+; db.sqlite.busytimeout=10s
+
+; Raw pragma option pairs to be used when opening the sqlite db. The flag
+; can be specified multiple times to set multiple options.
+; db.sqlite.pragmaoptions=auto_vacuum=incremental
+; db.sqlite.pragmaoptions=temp_store=MEMORY
+
 [bolt]
 
 ; If true, prevents the database from syncing its freelist to disk.