[plugin] director: make my first use of static_assert()'s.

feels very cool, so far. ;)
[plugins] director: fixes #44

PHP-FPM sends out EndRequest-records that inform the frontend that the
given request has been completed (in success or not).
However, a `phpinfo();` does not auto-generate a Status: response header
to tell us the response status code, and that's why we defaulted to 503
as the backend did NOT generate content (but did indeed set some
response headers).

Now, upon EndRequest, we test the protocol status and conditionally
set the response status code, if not yet set already.
[plugins] director: typo fixes
[plugins] director: also handle case of unknown protocol-status
[plugins] director: also delete road warrior on plugin destruction
[base] PerformanceCounter: Fixes bug in access out-of-bound data.

Following caused by a request to /server-status (status plugin) as the very
first request to the service.

==29198== Conditional jump or move depends on uninitialised value(s)
==29198==    at 0x8F6FB6A: x0::PerformanceCounter<900u, double>::average(unsigned int) const (PerformanceCounter.h:112)
==29198==    by 0x8F6CD9F: x0::HttpWorker::fetchPerformanceCounts(double*, double*, double*) const (HttpWorker.h:288)
==29198==    by 0x8F6E816: StatusPlugin::createResponseBody(bool) (status.cpp:257)
==29198==    by 0x8F6E5EF: StatusPlugin::handleRequest(x0::HttpRequest*, x0::FlowArray const&) (status.cpp:227)
==29198==    by 0x8F71143: void x0::HttpPlugin::handler_thunk<StatusPlugin, &(StatusPlugin::handleRequest(x0::HttpRequest*, x0::FlowArr
ay const&))>(void*, x0::FlowArray&, void*) (HttpPlugin.h:156)
==29198==    by 0x55C0F8D: x0::FlowBackend::Callback::invoke(int, x0::FlowValue*, void*) (in /home/trapni/projects/x0/lib/libXzeroFlow.
so.0.6.0)
==29198==    by 0x55C0BF1: x0::FlowBackend::invoke(int, int, x0::FlowValue*, void*) (FlowBackend.cpp:75)
==29198==    by 0x55C0E34: flow_backend_callback (FlowBackend.cpp:127)
==29198==    by 0x40525B9: ???
==29198==    by 0x514747C: x0::HttpWorker::handleRequest(x0::HttpRequest*) (HttpWorker.cpp:269)
==29198==    by 0x513FD13: x0::HttpConnection::onMessageHeaderEnd() (HttpConnection.cpp:362)
==29198==    by 0x5178655: x0::HttpMessageProcessor::process(x0::BufferRef const&, unsigned long*) (HttpMessageProcessor.cpp:893)
x0d: preserve log-target on SIGHUP reexec
x0d: fixes reexec on --instant mode wrt --log-file passing
x0d: also pass --pid-file=PATH to the new x0d if passed. Closes #46.
x0d: detach ownership from pid file in parent process on reexec
[plugins] director: debug code cleanup
[base] Url: added Url::parseQuery() and Url::decode() general purpose helper
[plugins] imageable: some brainstorming on this plugin
[base] SocketSpec: fixes operator==() for SocketSpec, which contained a little brain-dead line-exchange.
some repo cleanup
[flow] FlowRunner: implements int(expr) casts
ChangeLog updates
TODO updates ...
[base] Defines: adds likely/unlikely compiler helper
[base] BufferBase<>: fixes dump() always dumping 60 bytes instead of actual buf size