Accidental host merge lost 99 hours of work

[Thyme Lawn]

A user on Orbit@home ​accidentally merged 2 computers. The co-existent test wasn't performed as neither host had any credit.

The host retired by the merge had been running a task for 99 hours and this was aborted (as a detached client) when the server assigned a new host id on the host's next scheduler request.

This loss wouldn't have happened if there was a test that no tasks are in progress on hosts with no credit (in hosts_compatible()).

[davea]

(In [16163]) - web: in checking if hosts can be merged,

do time-overlap check if either host has pending results Fixes #748