Refactor async mode to no longer use zpty

See technique used in `fast-syntax-highlighting`:
- ca2e18bbc9
- http://www.zsh.org/mla/users/2018/msg00424.html

Also see http://www.zsh.org/mla/users/2018/msg00432.html

In async response handler:
- We only want to read data in case of POLLIN or POLLHUP. Not POLLNVAL
  or select error.
- We always want to remove the handler, so it doesn't get called in an
  infinite loop when error is nval or err.

There is an upstream bug that prevents ctrl-c from resetting the prompt
immediately after a suggestion has been fetched asynchronously. A patch
has been submitted, but a workaround for now is to add `command true`
after the exec.

See https://github.com/zsh-users/zsh-autosuggestions/issues/364

spec/integrations/client_zpty_spec.rb

@@ -1,10 +0,0 @@
-describe 'a running zpty command' do
-  let(:before_sourcing) { -> { session.run_command('zmodload zsh/zpty && zpty -b kitty cat') } }
-
-  it 'is not affected by running zsh-autosuggestions' do
-    sleep 1 # Give a little time for precmd hooks to run
-    session.run_command('zpty -t kitty; echo $?')
-
-    wait_for { session.content }.to end_with("\n0")
-  end
-end