Allow modification of agent setting

[andrewjstone]

We need to allow modification of the agent setting. The main reason is that we need to be able to open a large number of sockets with and without keepalive.

To me the most useful scenario seems to be:

• Set agent.maxSockets to a configurable high number
• at the end of each session pull that particular socket out of the pool so we can test load of connection setup/teardown as well