postgresql plugin: Don't do any SQL query when checking the connection.

First of all, this is not needed -- PQstatus() will correctly determine a
failed connection without that. Secondly, the "SELECT 42" that was used before
does not work, e.g. when querying PgBouncer (thanks to renchap for reporting
this on IRC!).
postgresql plugin: If a query fails, try to reconnect immediately.

Else, we would unnecessarily skip one iteration even when being able to
reconnect right away.
write_graphite plugin: Change <Carbon> blocks to <Node> blocks.

Many other write plugins, e.g. write_{mongodb,redis,riemann} use the
<Node "name"> syntax. This adds code to handle this syntax as well.

While the documentation doesn't mention it, the old <Carbon> syntax
(without a name) is still supported for backwards compatibility.
Merge remote-tracking branch 'github-tokkee/sh/postgresql'