dummynet: Move timekeeping information into dn_cfg

Just like with the packet counters move the timekeeping information into
dn_cfg. This reduces the global name space use for dummynet and will
make subsequent work to add vnet support and re-use in pf easier.

Reviewed by: donner
MFC after: 2 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Different Revision: https://reviews.freebsd.org/D29246