WIP on tim_server_tags
Tear out "datacenter" concept and replace it with "server tags".

Server tags can be set on the command line and altered at runtime
through a new table called `rethinkdb.server_config`. As of this
commit they don't have any effect, but that will change later.

This commit compiles but has not been tested.
Add a test for the server tag functionality, and make sure it passes.
Merge branch 'reql_admin' into tim_server_tags

Conflicts:
	src/clustering/administration/main/serve.cc
	src/clustering/administration/metadata.cc
Minor cosmetic changes.
Add RSI comment in metadata.cc.
Fix minor style issue in datum_adapter.hpp
Remove unnecessary r.literal() in test
Fix race condition in server_tags.py
Raise an error when trying to delete a missing row of a pseudo-table.