drivers: rmnet_shs: Add Max Backlog switch for TCP

Change instant rate timer to rely more on byte limit counts.
Add backlog limit to loaded core metric that will only be active
when a slow start flow is on the cpu. This type of NET_RX switch
will also have a longer core switch timer.

Change-Id: I414db0d10c1b72d54df25138bd8adf0902357847
Acked-by: Raul Martinez <mraul@qti.qualcomm.com>
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>