Auto reloading new versions of captive core relies on last modified timestamp which may be faulty

[tamirms]

In #3602 we implemented a feature in horizon which restarts captive core when a new version of captive core is detected on disk. The feature relied on last modified file timestamps to determine if the captive core binary was updated. However, when stellar core is installed via the debian packages the last modified timestamp of the binary is determined from the debian package changelog which is set to Nov 2018 for all versions of stellar core since 9.2.0-9.

Since the last modified timestamp is the same, horizon is not able to detect that captive core should be restarted. To make Horizon more robust to last modified timestamps we can instead use file hashes as the heuristic for determining if the captive core binary has been modified.

[tamirms]

Fixed in #4050