Performance degradation with Http2 client

The HTTP/2 client for internode communication has some serious bottleneck when the system is at a load. For the same scenario, the HTTP1 client performs much better.

Here's how to reproduce the test:

git clone https://github.com/fullstorydev/solr-bench
mvn clean compile assembly:single
./cleanup.sh && ./stress.sh http2-bug.json

^ This will run a stress test scenario using the HTTP1 client. After running this, change this line from http1=true to http1=false and re-run the test: https://github.com/fullstorydev/solr-bench/blob/master/http2-bug.json#L92

I ran this test on my AMD Ryzen 5700G, 64GB RAM machine. It has 8 cores/16 threads. The test with http1=true passes in about 30-40 minutes. The test with http1=false hangs indefinitely, with queries getting timed out all the time and the system experiencing some hang/stuck. All queries are sent to the node at port 50000, where we're changing the client type.

Test plan is here: https://github.com/fullstorydev/solr-bench/blob/master/http2-bug.json