Revert "Increase open file soft limit depending on pool size"

This reverts commit 2c98777.

We'll consider doing this in a better way in the future.

Check if process is running before killing on start-up failure (amphp#96

)

Remove unused import

Adjust AppVeyor to PHP 7.3

Cleanup test

Fix DefaultPool::shutdown()

Forgot to update this to match Worker::shutdown() behavior.

Store exit status promise

Return this promise from subsequent calls to shutdown().

v0.2.5

 - Fixed running inside PHARs if the PHAR doesn't have a `.phar` file extension.

Added signal() and getPid() methods to Process

v0.2.3

 - Fixed script path escaping in `Context\Process`, which fixes installations within paths with spaces. (amphp#38)