Server: Fixed handling of exceptions during packet constructions

Server: Fixed handling of exceptions during packet constructions

We fell into two LibUCW traps at once :)

First, fastbuf operations generate proper exceptions only when
the fastbuf is tied to a resource pool of the current transaction
by fb_tie(). Otherwise, they call plain die().

Second, fb-buf did not support bclose(), so it could not have been
tied.

LibUCW 6.5.5 fixes the second problem, so let us tie the fb-buf
properly.