Remove event listeners on pool/worker termination to remove circular references to callbacks

[lucasnetau]

While tracking down memory growth on a long running process using ReactPHP Filesystem I came across the following circular reference that needs to be cleaned up by the GC. Results where seen in benchmark/memory.php

In a small test of 2500 invocations of RPC this reduced GC cycles from 137 to 0

[WyriHaximus]

Just released v1.9.0 with this in it