multisocketsink: Map `GstMemory`s individually when sending

multisocketsink: Map `GstMemory`s individually when sending

If a buffer is made up of non-contiguous `GstMemory`s `gst_buffer_map`
has to copy all the data into a new `GstMemory` which is contiguous.  By
mapping all the `GstMemory`s individually and then using scatter-gather
IO we avoid this situation.

This is a preparatory step for adding support to multisocketsink for
sending file descriptors, where a GstBuffer may be made up of several
`GstMemory`s, some of which are backed by a memfd or file, but I think this
patch is valid and useful on its own.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=746150