handler.cache: Lazy opening of files and making of upstream connections

[mstoykov]

Situation:
4MB parts
1GB file
we have the first 200parts and the last 200MB will have to be retrieved from the upstream.

Currently we will open 200 files and open a connection to the upstream.
This is both wasteful and the upstream connection will timeout which because of #94 will mean the last 200MB will not be served.

Preferably a few parts should be opened at a time (the currently being transmitted and one upfront) and a connection to the upstream should be opened when it's needed or near then.