Add ConnectionReuseDemo

[rschmitt]

This example class is derived from a reproducer I wrote to investigate several reports I've received of a latency regression in the async client, which seems to be caused by increased TLS connection establishment. I've found scads of issues in just a few hours of messing with this thing, so I think it belongs in the repo.

[rschmitt]

@arturobernalg I have to hand it to you: of the three connection pool implementations, OFFLOCK is easily the best-behaved in the scenario this example class simulates. I found major defects in both LAX and STRICT:

• StrictConnPool: Fix FIFO implementation httpcomponents-core#609
• LaxConnPool: Loop over expired connections httpcomponents-core#610

[rschmitt]

The other major issue is that RequestConfig#getConnectionKeepAlive (or technically, whatever the ConnectionKeepAliveStrategy returns) appears to be the only config value that is threaded into PoolEntry or the connection pool. If instead you set a TTL or idle timeout on ConnectionConfig, it never propagates into those classes, which means they lease out expired connections without even realizing it, and then the connection manager has to deal with it (by establishing a new connection). It's possible that this is actually a regression introduced in a02455a.

[arturobernalg]

LGTM