multi: add block cache size config

This commit adds block cache size to the main lnd config along with the
chainreg config.

sample-lnd.conf

@@ -224,6 +224,12 @@
 ; The target location of the channel backup file.
 ; backupfilepath=~/.lnd/data/chain/bitcoin/simnet/channel.backup
 
+; The maximum capacity of the block cache in bytes. Increasing this will result
+; in more blocks being kept in memory but will increase performance when the
+; same block is required multiple times.
+; The example value below is 40 MB (1024 * 1024 * 40)
+; blockcachesize=41943040
+
 ; Optional URL for external fee estimation. If no URL is specified, the method
 ; for fee estimation will depend on the chosen backend and network. Must be set
 ; for neutrino on mainnet.