Update mongodb to 2.4.5.

For a full list of bugfixes, see the bugtracker:

https://jira.mongodb.org/secure/IssueNavigator.jspa?jqlQuery=project+%3D+SERVER+AND+fixVersion+in+%2812519%2C+12435%29
More reasonable DESCR
Merge remote-tracking branch 'origin/master' into joyent
Enable proper Git tag usage. To clone a single tag directly, make sure
to set GIT_TAG in your package Makefile (and beware that e.g. PKGVERSION
typically only resolves in mk/bsd.pkg.mk).