fix(nat): parse new show nat44 summary output

+ Keep support for VPP 22.02 CLI output.
 + Keep tolerating "timed out" sessions for 2202 output.
+ Subtract timed-out sessions for post-2202 output.
+ Explain the logic differences in docstring.
+ Convert the return value to int.

Change-Id: I8a119e1034848bfd0fb131cb633e2441b44634a2
Signed-off-by: Vratko Polak <vrpolak@cisco.com>