init: remove boost from ThreadImport.

[sinetek]

Problem
This is a refactor fix. We want to remove boost functions slowly.

Root Cause

Solution
We can work around the boost interruption point by just returning, and shutting down the program, instead. It is much cleaner than relying on boost to redirect control flow.

Unit Testing Results
Can be tested by calling -reindex or -loadblock and then pressing CTRL+C.

Should print something like:

...
2020-04-27T19:34:31Z [loadblk] Reindexing block file blk00005.dat...
^C2020-04-27T19:34:32Z [loadblk] Shutdown requested. Exit ThreadImport
2020-04-27T19:34:32Z [qt-init] Interrupting HTTP server
...

[CaveSpectre11]

Overall looks fine; but please maintain the bool type on LoadExternalBlockFile for future cleanup

[sinetek]

Travis early failed due to caching I believe..

[codeofalltrades]

Travis early failed due to caching I believe..
Yep, reran. All passing now.

[codeofalltrades]

utACK 3660e40
assuming the other reviewer's comments to corrected.

[sinetek]

Addressed the issues, ready for another review.

done

[CaveSpectre11]

utACK f2bbec8

[mimirmim]

utACK f2bbec8

[seanPhill]

ACK
With this PR #780 I've done numerous closes of the veil daemon (and of the Qt wallet) (with or without -reindex), with ctrl-c or otherwise and they all end without problem, with something like:

2020-09-15T00:00:38Z ThreadStaging exiting
2020-09-15T00:00:38Z tor: Thread interrupt
2020-09-15T00:00:38Z torcontrol thread exit
2020-09-15T00:00:38Z dnsseed thread exit
2020-09-15T00:00:38Z addcon thread exit
2020-09-15T00:00:38Z Shutdown: In progress...
2020-09-15T00:00:38Z UPNP_DeletePortMapping() returned: 0
2020-09-15T00:00:38Z upnp thread exit
2020-09-15T00:00:38Z msghand thread exit
2020-09-15T00:00:38Z net thread exit
2020-09-15T00:00:38Z opencon thread exit
2020-09-15T00:00:38Z Shutdown requested. Exit ThreadImport
2020-09-15T00:00:38Z ThreadStakeMiner() interrupted
2020-09-15T00:00:38Z ThreadStakeMiner exiting
2020-09-15T00:00:38Z scheduler thread interrupt
2020-09-15T00:00:38Z Shutdown: done