lncfg+scripts: use configurable global lock for postgres backends

Replace hardcoded WithGlobalLock assignment with configurable
options wallet postgres backends. Also add the WithGlobalLock
option to the channeldb table for postgres backends.

Defaults:
- channeldb: false (allow concurrent access)
- wallet: true (maintain safe single-writer behavior)

Users can now override these defaults via:
- db.postgres.channeldb-with-global-lock
- db.postgres.walletdb-with-global-lock

This gives operators flexibility while maintaining safe defaults
until full native SQL migration is complete.

Moreover exclude db.postgres.walletdb-with-global-lock check
in the sample config file script. We cannot easily check the
correct default because we set it later in the LND startup
sequence so we exclude it.

sample-lnd.conf

@@ -1625,6 +1625,17 @@
 ; Whether to skip executing schema migrations.
 ; db.postgres.skipmigrations=false
 
+; Use a global lock for channeldb access. This ensures only a single writer at
+; a time but reduces concurrency. This is a temporary workaround until the
+; revocation log is migrated to native SQL.
+; db.postgres.channeldb-with-global-lock=false
+
+
+; Use a global lock for wallet database access. This is a temporary workaround 
+; until the wallet subsystem is upgraded to a native sql schema.
+; db.postgres.walletdb-with-global-lock=true
+
+
 ; The maximum number of elements to use in a native-SQL batch query IN clause.
 ; db.postgres.query.max-batch-size=5000