Validate gRPC HPACK request fields

[xintenseapple]

Summary:

• Validate gRPC request HPACK field lengths before using fixed HPACK integer reservations for method, path, authority, configured headers, and pass-through headers.

Verification:

• git diff --check; ./auto/configure --with-http_v2_module --without-http_rewrite_module --without-http_gzip_module; make -f objs/Makefile -j2; objs/nginx -t -p "$PWD/" -c conf/nginx.conf

Fixes #28

[mdounin]

As demonstrated in #27, script length calculations might change over time even if all the variables are flushed. Since the checks added are not expected to be hit in practice and more like "last resort" checks to ensure there will be no overflow, it would be better to put the script-related checks where the actual serialization happens, to ensure all the possible issues are catched.

Placing relevant checks to where the actual serialization happens should also improve generated error messages (in the patch, most errors do not contain affected header names/values, as these are not available yet). Similarly, it would make it possible to get rid of seemingly unrelated changes in uri_len calculation, which are needed by the patch to correctly calculate length of the resulting URI. I've submitted a patch series which implements this approach.