realm: restore support for 2d/gather source data in gex active messages

[streichler]

The databuf-based approach has problems with flow control and should be disabled. Probably better to have the
data go inline in pktbufs.

[streichler]

Note: the current workaround of disabling databufs causes hangs with the ibv and ucx conduits when sysmem is successfully bound with gasnet and an inter-node copy is attempted in which the source instance is a external instance in sysmem. The hang can be worked around with -gex:bindhost 0 (i.e. disabling the binding of sysmem, which of course has a potential performance cost).