Title: Pre-establish upstream connections so that connections will be warmed up for future requests

Description:
Envoy can be deployed in use cases where establishing a new upstream connection is expensive w.r.t time (~300ms RTT). Since requests are typically queued for this duration, results in increased response times.

Proposal:
Pre-establish few upstream connections every time the number of free upstream connections falls below a specific threshold.

PS: I can work on this if it is reasonable.