switch from IO#sendfile_nonblock to IO#trysendfile
IO#trysendfile does not raise exceptions for common EAGAIN
errors, making it far less expensive to use with the following
concurrency models:
* Coolio
* CoolioFiberSpawn
* Revactor
* FiberSpawn
* FiberPool
This requires the new sendfile 1.1.0 RubyGem and removes support
for the sendfile 1.0.0. All sendfile users must upgrade or be
left without sendfile(2) support. IO#sendfile behaves the same
if you're using a multi-threaded concurrency option, but we
don't detect nor use it unless IO#trysendfile exists.