net: tcp: Define single config option for TIME_WAIT delay

Previously, there was a boolean CONFIG_NET_TCP_TIME_WAIT setting
("master switch") and numeric CONFIG_NET_TCP_2MSL_TIME setting,
both named not ideally (there were both NET_TCP_TIME_WAIT and
CONFIG_NET_TCP_TIME_WAIT symbols in the source, with very different
meaning; "2MSL_TIME" was also a roundabout way to refer to
TIME_WAIT state time). In addition to that, some code was defining
adhoc, hardcoded duplicates for these settings.

CONFIG_NET_TCP_2MSL_TIME was also measured in seconds, giving
poor precision control for this resource-tying setting.

Instead, replace them all with the single
CONFIG_NET_TCP_TIME_WAIT_DELAY setting, measured in milliseconds.
The value of 0 means that TIME_WAIT state is skipped.

Fixes: #7459

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>