rtt server: fix for dropped data when target has no space

rtt server: fix for dropped data when target has no space

rtt_write_channel may write less data than requested,
default device buffer size for channel 0 is 16 bytes,
so currently anything larger than this is dropped.

This fix implements per connection buffer and uses the
connection->input_pending flag to retry writes.

Change-Id: I00c845fccb0248550ad0f0fd9cda7bac7976b92b
Signed-off-by: fanoush <fanoush@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8360
Reviewed-by: zapb <dev@zapb.de>
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>