Fixed web runner just using a small limited number of connections
replaced the ThreadLocal with a LoadingCache
Added compact layout for Rest Runner and New Web Runner
SLT-488 Experimental asynchronous code for the new web runner
SLT-488 Fixed limitation on concurrent sessions
SLT-488 Adapted async web runner code to provide statistics correctly - refactoring and tests not done yet
SLT-488 Small optimisation on new webrunner
WebRunnerTests now working, thanks to adding FakeHttpAsyncClient.
Fixed some of the failing tests in RequestRunnerTest and removed some others.

The removed tests were very complex and not core functionality. They'll
most likely also be easier to implement on a higher abstraction level
(e.g. in WebRunnerTest).
Cleanup (removal of debug message and unused imports).
Extracted anonymous class to new name: WebRunnerByteConsumer
SLT-488 Fixed new webrunner running request - trying to remove wrong future
Merge branch 'async-web-runner' of https://github.com/SmartBear/loadui into async-web-runner
Merge branch 'async-web-runner' into async-web-runner-test-fixes
Misc small cleanups.
Refactored HttpWebResponse to make it immutable.
No longer reusing WebRunnerStatsSender.

I asked Renato about this. He didn't know what this was for and
suggested that I removed it.
WebRunnerStatsSender.resourceToVariableGroup is now thread-safe.
RequestRunner.pageRequest and RequestRunner.assetRequest is now thread-safe.
WebRunnerStatsSender now supports adding resources sequentially again.

I mistakenly broke it earlier in this PR.