souphttpsrc: add 'pause-mode' property

Submitted by Guillaume Desmottes @gdesmott

Link to original bug (#738298)

Description

The DLNA CVP-2 spec defines different 'pause' modes regarding the way client should handle the HTTP connection when being paused:

• stalling pause: keep the connection open and just stop reading from it. When resuming just resume reading from the connection. This is the current behavior in souphttpsrc.
• time/range pause: close the HTTP connection when pausing. When resuming re-establish a new HTTP connection and perform a time or range seek request to the server.

In order to support this new mode, we need some API controlling the behaviour of souphttpsrc when pausing.