Changing type from worker to supervisor

logplex_syslog_sup spawns a tcp_proxy_sup, but currently sets it as a
worker type of child. This patch fixes it by making it a supervisor.
Adds drain tokens to drain buffer overflows.

When drain buffers overflow (both on TCP/syslog and HTTP/syslog), the
error message doesn't include the drain token, but only the default
'logplex' value:

    ... logplex/logplex: Error L10 (Drain buffer overflow) -> ...

This patch adds the drain token to the buffer overflow messages for TCP
and HTTP drains so that user can figure out which drain has problems.