Memory allocated for outbound SVC PDUs may not be freed

As reported on the mailing list, depending on the version of FreeRDP installed, the wStream allocated for transmission along a static virtual channel may not be freed:

https://lists.apache.org/thread.html/r39f830253413b2143438d09533c59e193e78729ef036bf1a1356f33b%40%3Cuser.guacamole.apache.org%3E

Possible solutions include:

• Explicitly freeing the wStream (if the version of FreeRDP present at build time will not do so)
• Leveraging the wStream pool that will be automatically freed (if the version of FreeRDP present at build time supports this)

Unfortunately, even after this is addressed, it may always be the case that a build of guacamole-server against FreeRDP 2.0.0 will suddenly start leaking memory if FreeRDP is updated and guacamole-server is not rebuilt, as the difference in behavior will only be detectable at build time.