Invalidate file list cache when cache file has a future last modified time

[meaksh]

What does this PR do?

This PR fixes an issue in fileserver.check_file_list_cache which happens if the last modification time for the file list cache file is set to a future time.

When this situation happens, the cache file is always taken and never* refreshed. Therefore, new files placed on the "file_roots" are not visible for the minions.

Previous Behavior

We see these logs and cache file is NOT refreshed:

2020-09-22 13:36:48,228 [salt.fileserver  :143 ][DEBUG   ][11349] Cache file modified time is in the future, ignoring. file=/var/cache/salt/master
/file_lists/roots/base.p mtime=1600774608 current_time=1923304320
2020-09-22 13:36:48,229 [salt.fileserver  :159 ][DEBUG   ][11349] Returning file list from cache: age=0 cache_time=20 /var/cache/salt/master/file_
lists/roots/base.p

New Behavior

We see these logs and cache file is refreshed:

2020-09-22 13:37:42,475 [salt.fileserver  :143 ][DEBUG   ][12474] Cache file modified time is in the future, ignoring. file=/var/cache/salt/master
/file_lists/roots/base.p mtime=1600774662 current_time=1923304320
2020-09-22 13:37:42,475 [salt.fileserver  :153 ][WARNING ][12474] The file list_cache was created in the future!

Merge requirements satisfied?

• Docs
• Changelog - https://docs.saltstack.com/en/master/topics/development/changelog.html
• Tests written/updated

Commits signed with GPG?

Yes

[meaksh]

@Ch3LL @s0undt3ch this PR has been here for a while and it's ready to be merged.

Would you mind to take a look? Thanks!

[s0undt3ch]

It's now properly tagged to go into the next major release.
It will get merged.