inline repository remote status thread may be randomly terminated

When a content request fails outbound for a proxy repository in Nexus 3, a daemon thread is launched to attempt rechecking the remote host to determine if the repo can be moved back out from auto-block state.

This daemon thread may make anywhere from 1-4 attempts using a HEAD request to the root url ( at / ) of the configured remote url. Then the thread will disappear and no more automated checks will be performed. It is acting like the thread is garbage collected by the process reaper.

Expected:

• there should be a long lived check that does not disappear as long as a repo is auto blocked
• the request it makes should not be at the root of the server - it should be at the root of the remote url
• there should be an INFO log message whenever a status check is performed ( see NEXUS-12077 )