[http] HttpWorker: replacing locking message queue with The lock-free Queue.
[base] added support for multi accept().

When compiled with -DWITH_MULTI_ACCEPT=ON, you can pass multi_accept
option to a listener (e.g. listen and ssl.listen) with the number
of maximum attempts to accept() an incoming connection (defaults to 1).
If the accept() syscall returns EAGAIN the multi-accept loop will
be left early.

This change should help performance on servers with very high number
of incoming connections per second.

WITH_MULTI_ACCEPT defaults to OFF.
[http] HttpConnection: added support for opportunistic write()'s.

This enables support for opportunistic writes of the HTTP response
message to the client, that is, if enabled at compile-time via
-DWITH_OPPORTUNISTIC_WRITE=ON, then it is always first attempted
to transmit the response message (chunk) directly to the client,
ideally writing everything, but if it would block, then the write
will be delayed until a non-blocking write() to the client's socket
can be guaranteed.

WITH_OPPORTUNISTIC_WRITE defaults to OFF.
[http] HttpConnection: fixes bug with opportunistic write and TCP_DEFER_ACCEPT enabled at the same time.

When TCP_DEFER_ACCEPT and opportunistic writes are enabled at compile
time, it may happen that the whole request processing can be done
in a non-blocking manner - regarding accept() + read() + write().

However, when this happens, the next request (e.g. when keep-alived)
has not been processed because nobody told the client socket to watch
for new input, which is now fixed by emulating the ev::READ event
within the start() call instead of directly and just calling readSome().
ChangeLog: wording fixes [ci skip]
performance.conf tweaks, and tcp_cork fix (must of course be set to true)
added support for setting SO_LINGER
trying to make travis-ci happy
travis-ci....
travis-ci....
travis-ci....
[flow] added support for using ':' with symbol as LHS instead => with type string as LHS. closes #55
[plugins] removed plugins fastcgi & proxy as they're deprecated by director plugin.
[base] Actor: use lock-free Queue over std::deque.
[http] HttpConnection: minor code cleanup
[io] FileSink: added ability to cycle file sink's underlying file descriptor
[io] added SyslogSink API
x0d: let x0d initialize syslog target properly
[plugins] accesslog: added support for logging into syslog target. Closes #31.
[plugins] accesslog: fixes compile error