Bring in mod_http2 v2.0.4 from github

[icing]

The 2.4.x/trunk and standalone mod_h2 sources have diverged while the pollset changes have been verified in several installations from the mod_h2 github repository.

The current state of trunk with its new request/response meta bucket is not suitable for backport into 2.4.x. But with reports of people using the mod_h2 in production, I feel comfortable to bring these changes into the 2.4.x branch.

Changes from the github mod_h2 repository:

• new directive H2HeaderStrictness (highest|rfc7540|rfc9113) to configure the strictness of HTTP header checks. rfc9113 forbids leading and trailing whitespace in headers and is supported when building against nghttp2 v1.50.0 or newer. The default is the backward compatible behavior, e.g. rfc7540.
• new directive 'H2StreamTimeout' to configure a separate value for HTTP/2 streams, overriding server's 'Timeout' configuration. [rpluem]
• HTTP/2 connections now use pollsets to monitor the status of the ongoing streams and their main connection, if the platform supports them and APR >= 1.6.0 is in play.
• H2SerializeHeaders no longer has an effect. A warning is logged when it is set to "on". The switch enabled the internal writing of requests to be parsed by the internal HTTP/1.1 protocol handler and was introduced to avoid potential incompatibilities during the introduction of HTTP/2.
• Removed the abort/redo of tasks when mood swings lower the active limit. This probably hurts the server more than it helps. Some setup, like proxied connections, can be slow to respond to the abort of a connection.
• a bug that prevented trailers (e.g. HEADER frame at the end) to be generated in certain cases was fixed. See #233 where it prevented gRPC responses to be properly generated.