If shared queue is empty, allocated read buffer should be explicitly
released.

Found by Coverity (CID 363943).
The issue was introduced in f5ba5973a0a3.

Removing unnecessary context operations from shared queue processing loop.
Initializing temporary queues only when required.

The issue occurred under highly concurrent request load in Go applications.
Such applications are multi-threaded but use a single libunit context; any
thread-safe code in the libunit context is only required for Go applications.

As a result of improper request state reset, the recycled request structure was
recovered in the released state, so further operations with this request
resulted in 'response already sent' warnings.  However, the actual response was
never delivered to the router and the client.

The issue only occurred in Go applications because "port_send" is overloaded
only in Go.  To reproduce it, send multiple concurrent requests to the
application after it has initialised.  The warning message "[unit] [go] port
NNN:dd not found" is the first visible aspect of the issue; the second and more
valuable one is a closed connection, an error response, or a hanging response to
some requests.

When the application starts, it is unaware of the router's worker thread ports,
so it requests the ports from the router after receiving requests from the
corresponding router worker threads.  When multiple requests are processed
simultaneously, the router port may be required by several requests, so request
processing starts only after the application receives the required port
information.  The port should be added to the Go port repository after its
'ready' flag is updated.  Otherwise, Unit may start processing some requests and
use the port before it is in the repository.

The issue was introduced in changeset 78836321a126.

Debug logging depends on macros defined in nxt_auto_config.h.