Request pool should be configurable

[ravi]

Currently, the pool size used by the underlying request module is hard-coded to 100.

node-etcd/src/client.coffee

Line 11 in 9c14f0a

options.pool = maxSockets: 100

Since EtcD is built to handle thousands of clients, in certain scenarios (including mine) this can result in 100k+ open connections on the EtcD servers. It would be helpful to keep this a configurable option rather than hard-coding it.