perf(gthread): Use request timeout / 2 for epoll timeout

gthread calls epoll_wait (and 2 other syscalls) every second because it
specifies timeout to be 1 second.

```
λ sudo strace -p `pgrep -f "gunicorn: worker" | head -n1`
strace: Process 30815 attached
epoll_wait(7, [], 1, 666)               = 0
getppid()                               = 30800
utimensat(6, NULL, [{tv_sec=3157, tv_nsec=198136276} /* 1970-01-01T06:22:37.198136276+0530 */, {tv_sec=3157, tv_nsec=198136276} /* 1970-01-01T06:22:37.198136276+0530 */], 0) = 0
epoll_wait(7, [], 1, 1000)              = 0
getppid()                               = 30800
utimensat(6, NULL, [{tv_sec=3158, tv_nsec=204192934} /* 1970-01-01T06:22:38.204192934+0530 */, {tv_sec=3158, tv_nsec=204192934} /* 1970-01-01T06:22:38.204192934+0530 */], 0) = 0
epoll_wait(7, [], 1, 1000)              = 0
getppid()                               = 30800
utimensat(6, NULL, [{tv_sec=3159, tv_nsec=210145196} /* 1970-01-01T06:22:39.210145196+0530 */, {tv_sec=3159, tv_nsec=210145196} /* 1970-01-01T06:22:39.210145196+0530 */], 0) = 0
epoll_wait(7, [], 1, 1000)              = 0
getppid()                               = 30800
utimensat(6, NULL, [{tv_sec=3160, tv_nsec=215517372} /* 1970-01-01T06:22:40.215517372+0530 */, {tv_sec=3160, tv_nsec=215517372} /* 1970-01-01T06:22:40.215517372+0530 */], 0) = 0
epoll_wait(7, ^Cstrace: Process 30815 detached
 <detached ...>
 ```

Timing out every second wakes up the process and loads it on CPU even
if there is nothing to service.

This can be detrimental when you have total workers >> total cores and
multi-tenant setup where certain tenants might be sitting idle. (but not
"idle enough" because of 1s polling timeout)

This can possibly keep a completed future in queue for a small while,
but I don't see any obious problem with it except few bytes of extra
memory usage. I could be wrong here.

fixes #3317

gunicorn/workers/gthread.py

@@ -207,7 +207,7 @@ def run(self):
             # can we accept more connections?
             if self.nr_conns < self.worker_connections:
                 # wait for an event
-                events = self.poller.select(1.0)
+                events = self.poller.select(self.timeout)
                 for key, _ in events:
                     callback = key.data
                     callback(key.fileobj)