until it's been allocated.

This patch delays the VXID allocation until the time we know that we
will actually be logging anything for this request (not completely all
whitespace noise). This keeps the code in one place only (plus the ESI
mock request handling), instead of separated between the linger wait,
cache acceptor and pipelining code paths.

Coincidentally fixes the logging of timestamps problem in #1588.

Test case by daghf.

Fixes: #1581 #1588

Conflicts:
	bin/varnishd/cache/cache_esi_deliver.c
	bin/varnishd/cache/cache_fetch.c
	bin/varnishd/cache/cache_http1_fsm.c
	bin/varnishd/cache/cache_session.c