Made the test more reliable.
Updated flow control implementation to detect when senders exceed
allowed windows.
Introduced configuration properties for ALPN/NPN advertised protocols.

No more need to copy protonego-alpn.xml to a jetty.base to specify
which protocols are advertised and in which order.
Instead, simply specify:

alpn.protocols=h2-14,http/1.1
alpn.defaultProtocol=http/1.1

in start.ini (in the example above for http2).
Refactored host and port to local variables.