[SECURITY] Protect persisted session IDs from being used directly

Instead of storing session IDs with their corresponding storage
backends in plain text, their HMAC-SHA256 (Redis) or HMAC-MD5 (DB)
is being used. HMAC-MD5 had to be chosen to avoid breaking changes
for limited field size in database fields (32 characters currently).

This change also allows a fallback to non-hashed-session values,
meaning that
* set() and update() will create new session records with the hashed
  identifier
* get() contains a fallback to the non-hashed-version when no session
  with a hashed identifier is found

Resolves: #91854
Releases: master, 10.4, 9.5
Change-Id: Ia57acc5e0d0cf71088af1aaff1ab894bd1d4e3dd
Security-Bulletin: TYPO3-CORE-SA-2020-011
Security-References: CVE-2020-26228
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/66664


Tested-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>