Disable pool_connection () for stopped bootstrap or bootstrap_client

[SergiySW]

Fixes #1496

[zhyatt]

@SergiySW Can you find out when this issue first showed up? And also if anything we have done recently has made this worse? Trying to evaluate where to include this fix.

[SergiySW]

Ptobably it's related to lazy_bootstrap implementation (#1332) especially https://github.com/nanocurrency/raiblocks/blob/master/rai/node/bootstrap.cpp#L594
But it also happening with previous versions or with disabled lazy bootstrap, just less often.
Also bulk_pull_client and pulk_push_server now more often drop connections

[zhyatt]

Looking at including this in V18 unless there is an urgency to push earlier.

[rkeene]

This ensures that we respect the request to stop using a connection and close it as soon as it goes out of scope -- looks good to me !