[CORE] net: Improve netswitch bottom-half thread

This patch does two improvements for the bottom-half thread of
netswitch. This are as follows:

1. The thread should not wait infinitely for xfer request. Instead,
it should wake periodically after sufficently long time to check
for available xfer request.

2. We should not hold xfer request instance till mbuf is being process.
The xfer request should be freed immediately so that more xfer request
can be submitted.

Signed-off-by: Anup Patel <anup@brainfault.org>