Differentiate between CURLOPT_TIMEOUT and CURLOPT_CONNECTTIMEOUT

Problem/Motivation

It would be great if the api would allow to set distinct values for CURLOPT_TIMEOUT and CURLOPT_CONNECTTIMEOUT.
This allows to quickly skip requests that can't connect (CURLOPT_CONNECTTIMEOUT) an thus aren't worth waiting for - while waiting quite long when it's worthwile (CURLOPT_TIMEOUT) e.g. because the processing of a request takes quite a while.

Proposed resolution

Add the new option connection_timeout. If the option isn't set explicitly fallback to the execution timeout - that way it's fully backward compatible.

Remaining tasks

Reviews needed.

User interface changes

None.

API changes

New option, fully backward compatible.

Data model changes

None.