Revert "Add agentkeepalive as default HTTPS agent"

This reverts commit 214fedfe66c41e15666c5a65bc3cb08bca9fd1bf.

This module needs more investigation before it can be re-introduced
into the SDK since it causes various "hang" and socket leak issues
(see below).

Fixes #297, #310
Remove unused operation tests
Abort stream on timeout