Fixes to prevent race conditions

This release fixes a number of concurrency issues.

2020-08-06 update findVirtualIP (commit#: a6d7ae3)
2020-08-06 Add ClaimIP() function (commit#: bcf6077)
2020-08-05 test for invalid arp packet logging (commit#: ae8e48e)
2020-08-05 respond with announcement if IP is virtual (commit#: 118428d)
2020-08-05 print table when ip goes offline (commit#: 3b1cd21)
2020-08-05 minor code cleanup to updateIP signature (commit#: 0752109)
2020-08-05 fix lock() in whois (commit#: e0471e4)
2020-08-05 fix lock() in IPChange (commit#: 4832ef2)
2020-08-05 fix lock() in handler (commit#: e90413e)
2020-08-05 fix bug in handler (commit#: 52494cd)
2020-08-05 change findIP signature (commit#: 487ee51)
2020-08-04 upgrade mdlayher (commit#: caf03e9)
2020-08-04 update go.mod (commit#: 75d12fe)
2020-08-04 mod tidy on arm (commit#: a1bbec0)
2020-08-04 keep router mac (commit#: fa6e61d)
2020-08-04 add GetTable() (commit#: c6f04ec)