kgio 2.11.0 - reverting 2.10.0 breakage

Mainly this release reverts back to kgio 2.9.3 behavior by
supporting "autopush" functionality in the C extension.
Removing this feature in 2.10.0 was a monumental mistake(*) on
my part which caused a performance regression for ONE known user
of kgio on the deprecated (but supported) Rainbows! server.

This release does not affect known users outside of Rainbows!,
but we may have unknown users, too.

The attempt at a pure Ruby version of autopush had flaws and
regressions in real-world testing, so that was abandoned.

So yeah, kgio will be supported indefinitely even in the face
of internal changes to MRI, but I strongly discourage its use
for new projects.
Users who only support Ruby 2.3 (and later) are encouraged to
use `exception: false` with all *_nonblock methods.

4 changes since 2.10.0:

      resurrect Kgio.autopush support in pure Ruby [REVERTED]
      reinstate the original (and dangerous) autopush in C
      TODO: update with Ruby 2.3 status
      test: increase test data sizes to fill socket buffers

(*) - as was making this a separate project, at all