[common/desktop] Thread/Threadpool consolidation

[gubatron]

Open an experimental branch on which all ThreadPools, ExecutorService instances, ThreadExecutor, ThreadPoolExecutor are all managed by a single entity in charge of 2 or 3 threadpools, and curb the creation of threads.

We've already reduced thread stack with -Xss, 256k for macos, 196k for windows, Linux tests still pending.

The goal is to reduce thread usage to a minimum necessary and memory consumption for better performance.

[gubatron]

Some progress:

SearchManager 08effd5

HttpDownload 883d4ca

[gubatron]

2.1.9 builds address a lot of these issues for Android, but there's still over 100 threads out there