GUACAMOLE-1170: Use min_size with user stream pool to delay stream index reuse.

[sschiffli]

Adding minimum size to user stream pool, so new stream integers are preferred over reused integers until the minimum size is reached.

This fixes a race condition with stream index reuse outlined in GUACAMOLE-1170.

[mike-jumper]

I think the same changes you've made to the user-specific stream pool also need to be made to the connection-wide stream pool:

guacamole-server/src/libguac/client.c

Lines 284 to 285 in cf09d3a

	/* Allocate stream pool */
	client->__stream_pool = guac_pool_alloc(0);

Other than that, LGTM. Really glad this didn't require changes to guac_pool.

[sschiffli]

Since client streams don't receive ack's I wasn't sure if it was worth making the change there. I certainly could for consistencies sake but it's not clear if the same bug could affect client streams too.

[mike-jumper]

Good point. I think you're right that the same ambiguity doesn't apply to connection-wide streams.