libalias: Promote per instance global variable timeStamp

Summary:

• Use LibAliasTime as a real global variable for central timekeeping.
• Reduce number of syscalls in user space considerably.
• Dynamically adjust the packet counters to match the second resolution.
• Only check the first few packets after a time increase for expiry.

Discussed with: hselasky
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D30566