GUACAMOLE-1974: Allow deferring received pipe streams for later consumption by name-specific handlers.

[jmuehlner]

This change modifies the ManagedClient class such that pipe streams prior to an onpipe handler being set can still be processed.

The idea is that you can register a pipe stream handler for a particular pipe stream name, and that handler will be reliably invoked when a new pipe stream with that name is received, OR if such a pipe stream was already received prior to registration.