Log when MAP_POPULATE gets disabled

[digivava]

Description

This PR adds a log line during the log store and FSM setup process that warns when Linux's MAP_POPULATE flag for mmap has been disabled. (That flag populates page tables ahead of time before we try to open a database file, which helps limit page faults and thus speeds up the process.)

Without this log message, the Vault process appears to hang for a long period of time while it tries to open up a large database. This log line will help warn users so they don't just think Vault is being broken and unresponsive.

Note that the unix.MAP_POPULATE flag is only available on Linux, so Go won't compile if I reference it in files that don't end in _linux.go, hence the slight gymnastics.

I've also added some comments to help noobs to these concepts like myself understand how this flag is used.

TODO only if you're a HashiCorp employee

• [N/A] Backport Labels: If this PR is in the ENT repo and needs to be backported, backport
  to N, N-1, and N-2, using the backport/ent/x.x.x+ent labels. If this PR is in the CE repo, you should only backport to N, using the backport/x.x.x label, not the enterprise labels.
  • [N/A] If this fixes a critical security vulnerability or severity 1 bug, it will also need to be backported to the current LTS versions of Vault. To ensure this, use all available enterprise labels.
• ENT Breakage: If this PR either 1) removes a public function OR 2) changes the signature
  of a public function, even if that change is in a CE file, double check that
  applying the patch for this PR to the ENT repo and running tests doesn't
  break any tests. Sometimes ENT only tests rely on public functions in CE
  files.
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• [N/A] RFC: If this change has an associated RFC, please link it in the description.
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[github-actions]

CI Results:
All Go tests succeeded! ✅

[github-actions]

Build Results:
All builds succeeded! ✅

[digivava]

I added this to match the comment up above where we explain that we're creating the FSM. Helps people like me who are new to the concept that Vault has two different databases that need to be created.

[raskchanky]

Looks good! Just a few small comments about one of the tests.

[raskchanky]