database: separate transaction-handling and retry-handling.

Previously call-with-transaction would both retry when SQLITE_BUSY errors were
thrown and do what its name suggested (start and rollback/commit a
transaction).  This changes it to do only what its name implies, which
simplifies its implementation.  Retrying is provided by the new
call-with-SQLITE_BUSY-retrying procedure.

* guix/store/database.scm (call-with-transaction): no longer restarts, new
  #:restartable? argument controls whether "begin" or "begin immediate" is
  used.
  (call-with-SQLITE_BUSY-retrying, call-with-retrying-transaction,
  call-with-retrying-savepoint): new procedures.
  (register-items): use call-with-retrying-transaction to preserve old
  behavior.

* .dir-locals.el (call-with-retrying-transaction,
  call-with-retrying-savepoint): add indentation information.

.dir-locals.el

@@ -90,7 +90,9 @@
    (eval . (put 'with-database 'scheme-indent-function 2))
    (eval . (put 'call-with-transaction 'scheme-indent-function 2))
    (eval . (put 'with-statement 'scheme-indent-function 3))
+   (eval . (put 'call-with-retrying-transaction 'scheme-indent-function 2))
    (eval . (put 'call-with-savepoint 'scheme-indent-function 1))
+   (eval . (put 'call-with-retrying-savepoint 'scheme-indent-function 1))
 
    (eval . (put 'call-with-container 'scheme-indent-function 1))
    (eval . (put 'container-excursion 'scheme-indent-function 1))