Update UndertowHttpHandlerAdapter to dispatch

[rstoyanchev]

As per the discussion under https://issues.redhat.com/browse/UNDERTOW-2512, UndertowHttpHandlerAdapter needs to start async handling explicitly by calling dispatch.

[philwebb]

Will this one be backported as some point?

[simonbasle]

The plan is to backport it to 6.1.x at some point indeed. We want to be a tiny bit more cautious with upgrading the Undertow baseline in the existing 6.1.x line, and delaying the backport could be an opportunity to gather feedback on the change from 6.2.

[simonbasle]

@philwebb this has now been backported to 6.1.x, available in 6.1.16 snapshots 👍