Fixed shared memory writing and fork losing received messages

[ghola]

Two issues fixed:

1. The Fork was losing received messages. When waiting it does a ::receive and does nothing with the result. The fix was to add a messages collection to the form and have the ::receive populate that collection. This way messages can be retrieved at any time using ::getMessages.
2. Shared memory was creating imbricated arrays with each ::send making the Fork::receive end up with array in array in array, etc. instead of a flat array of messages.

[ghola]

Getting the builds to pass seems to be a matter of luck, even on the master. They randomly fail for various php versions but always on the same test: ProcessManagerTest::testLargeBatchProcessing.