lncfg+channeldb: add autocompact flags to BoltConfig

sample-lnd.conf

@@ -928,3 +928,15 @@ litecoin.node=ltcd
 [bolt]
 ; If true, prevents the database from syncing its freelist to disk. 
 ; db.bolt.nofreelistsync=1
+
+; Whether the databases used within lnd should automatically be compacted on
+; every startup (and if the database has the configured minimum age). This is
+; disabled by default because it requires additional disk space to be available
+; during the compaction that is freed afterwards. In general compaction leads to
+; smaller database files.
+; db.bolt.auto-compact=true
+
+; How long ago the last compaction of a database file must be for it to be
+; considered for auto compaction again. Can be set to 0 to compact on every
+; startup. (default: 168h)
+; db.bolt.auto-compact-min-age=0