New optional configuration parameter introduced: limits.reschedule_timeout.
Default value 1 second. In the case when request is written to the port
socket 'in advance', it is called 'pending'.
On every completed request, the head of pending request is checked against
reschedule timeout. If this request waiting for execution longer than
timeout, it is cancelled, new port selected for this request.
Application timeout limits maximum time of worker response in processing
particular request. Not including the time required to start worker,
time in request queue etc.
This patch increase precedence of non-started worker over busy worker.
1. idle worker;
2. start new worker;
3. busy worker, but can accept request in advance;
The level hash uses the NULL value as the indicator of a free entry in a bucket.
So, inserting a NULL value breaks the hash and can lead to a bucket overflow.
In case of the collision counter, the value wasn't initialized, since it's not
needed for the purpose of checking collisions. As a result, it might contain
any garbage from the stack and in some rare cases the value was NULL.
Now the value is initilized.
To communicate with the Go program, Unit setup environment variable
named NXT_GO_PORTS with value contains Unit version, stream id to confirm
application is started, and Unit ports information. Go Unit package parses
this string and compares runtime version with compile time version. In case
of parse error or version mismatch, ListenAndServe() returns with the error.
Passing unsafe.Pointers (void *) from Go to C is complicated by an attempt
to make such pointers less unsafe.
A straightforward optimization is to replace 'unsafe.Pointer' with 'uintptr'
(thanks to Xin Huang for the idea: https://stackoverflow.com/a/44826533 ).
As a result, request registry with mutex is gone.
Go request registration should be removed before C request memory freed.
C request address used as a key in Go map. Freed memory can be instantly
reused for other request and older request registration should removed at this
point to avoid collisions.
