database: pull out database code into a new module

We're going to reuse the database controllers for the accounting plugin

db/exec.h

@@ -0,0 +1,52 @@
+#ifndef LIGHTNING_DB_EXEC_H
+#define LIGHTNING_DB_EXEC_H
+#include "config.h"
+
+#include <ccan/short_types/short_types.h>
+#include <ccan/take/take.h>
+
+struct db;
+
+/**
+ * db_set_intvar - Set an integer variable in the database
+ *
+ * Utility function to store generic integer values in the
+ * database.
+ */
+void db_set_intvar(struct db *db, char *varname, s64 val);
+
+/**
+ * db_get_intvar - Retrieve an integer variable from the database
+ *
+ * Either returns the value in the database, or @defval if
+ * the query failed or no such variable exists.
+ */
+s64 db_get_intvar(struct db *db, char *varname, s64 defval);
+
+/* Get the current data version (entries). */
+u32 db_data_version_get(struct db *db);
+
+/* Get the current database version (migrations). */
+int db_get_version(struct db *db);
+
+/**
+ * db_begin_transaction - Begin a transaction
+ *
+ * Begin a new DB transaction.  fatal() on database error.
+ */
+#define db_begin_transaction(db) \
+	db_begin_transaction_((db), __FILE__ ":" stringify(__LINE__))
+void db_begin_transaction_(struct db *db, const char *location);
+
+bool db_in_transaction(struct db *db);
+
+/**
+ * db_commit_transaction - Commit a running transaction
+ *
+ * Requires that we are currently in a transaction.  fatal() if we
+ * fail to commit.
+ */
+void db_commit_transaction(struct db *db);
+
+
+#endif /* LIGHTNING_DB_EXEC_H */