http2: reassemble entity bodies in data frames

http2: reassemble entity bodies in data frames

This commit reassembles data frames to build up the full entity body. It does
this for both client/server request and responses. Additionally, it also
decompresses bodies if they have the correct content-encoding header provided
and are not partial bodies.

Bug: 13543
Change-Id: I1661c9ddd09c1f6cf5a08b2b1921f95103aebb52
Reviewed-on: https://code.wireshark.org/review/20737
Petri-Dish: Anders Broman <a.broman58@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Anders Broman <a.broman58@gmail.com>