utils: Add critical sections.

* src/cuirass/utils.scm (make-critical-section)
(call-with-critical-section): New procedures.
(with-critical-section): New macro.
* src/cuirass/http.scm (with-database-access): Remove.
(handle-build-request, handle-builds-request, url-handler): Use
'with-critical-section' instead of 'with-database-access'.
(run-cuirass-server): Remove 'spawn-fiber' call.  Use
'make-critical-section' instead.