services/horizon: Fix captive-core's reset of its tmp dir

[2opremio]

What

Fix how the Captive Core backend resets its temporary directory. Previously:

• it removed its contents, but didn't reset its reference to the path.
• it didn't re-create the temporary directory after close()

Why

It made Captive core enter an error loop after a reset was attempted (e.g. after encountering an unexpected state).

Fixes #3145

Known limitations

It may be cleaner to just create another runner instead of just expecting stellarCoreRunner to work after a call to close() (which is how it is today).

[2opremio]

It may be cleaner to just create another runner instead of just expecting stellarCoreRunner to work after a call to close() (which is how it is today).

It is much cleaner (and there was a bug preventing this from happening).

I created #3162 , which overrides this PR (sorry @bartekn )