https://github.com/varnishcache/varnish-cache/pull/4013 fixes two
issues in Varnish-Cache, which are relevant for SLASH/fellow and of
which the first is the root cause of #33.

This commit works around these issues until the fix gets merged:

Because of the wrong use of the .objtrimstore API function by
varnish-cache, we remove it from our obj_methods and exploit the fact
that varnish-cache always sets the OA_LEN attribute when the object is
complete: We move the trimstore function there, effectively calling it
at the right time only.

The inefficient memory allocation fixed in the second commit of
VC#4013 is particularly relevant for fellow, because it causes the
allocation code to assume that the object might grow up to the maximum
possible size, which causes a substantial over-allocation. We work
around this issue for the case that a 304 copy is made from fellow to
fellow by using private thread-local storage to emulate basically the
same function as the #4013 fix.

Closes #33
Ref https://github.com/varnishcache/varnish-cache/pull/4013