riemann plugin: Fix a double-free issue with the shared structure.

riemann plugin: Fix a double-free issue with the shared structure.

The reference to the riemann_host structure is shared between the write
and the notification callback. Upon exit, riemann_free() is called twice,
via the user_data_t structure passed to the register functions. Previously
this would have caused a double-free, which is unacceptable.

This patch introduces a reference counter and will only free the structure
(and close the socket) when the last reference is freed.

The locking has been moved out of the riemann_connect() function and into
the riemann_send() function. It is unlikely, but not impossible that two
threads interfere with each other when writing to the socket.