Revert "kvdb/postgres: remove global application level lock"

This reverts commit 43a1ca4f3d.

kvdb/sqlbase/db.go

@@ -55,6 +55,10 @@ type Config struct {
 	// commands. Note that the sqlite keywords to be replaced are
 	// case-sensitive.
 	SQLiteCmdReplacements SQLiteCmdReplacements
+
+	// WithTxLevelLock when set will ensure that there is a transaction
+	// level lock.
+	WithTxLevelLock bool
 }
 
 // db holds a reference to the sql db connection.
@@ -75,6 +79,10 @@ type db struct {
 	// db is the underlying database connection instance.
 	db *sql.DB
 
+	// lock is the global write lock that ensures single writer. This is
+	// only used if cfg.WithTxLevelLock is set.
+	lock sync.RWMutex
+
 	// table is the name of the table that contains the data for all
 	// top-level buckets that have keys that cannot be mapped to a distinct
 	// sql table.