Allow socket pool size randomization for proxies

Socket pool size randomization is a security mechanism that prevents attackers from exploiting deterministic capacity limits to learn cross-site information.
If the capacity for a pool is normally 128 sockets, this mechanism randomly caps the pool between 128 and 256.
This can allow up to 2x as many connections to the proxy, but in practice the expected value is more like 1.2x.

This impacts the settings from MaxConnectionsPerProxy and MaxConnectionsPerProxyForWebSocket.
Instead of them defining the upper limit, the upper limit is 2x their values (though again, the expected value in practice is more like 1.2x them).

This is enabled by default for all pools, but this policy allows the feature to be disabled for proxy pools specifically.