riemann plugin: Clean up the riemann_connect() function.

Don't warn when connecting to some address fails. The system could, for
example, not have any IPv6 connectivity but the address record also
includes an IPv6 address. In this case a connect(2) call will fail but
this is not a problem.

Due to the new locking one of the concurrency checks was made redundant.
write_riemann plugin: Rename the "riemann" plugin.
write_riemann plugin: Ensure a unified logging prefix.