add optional cache for login result and htpasswd + fixes

[pbiering]

• Add: option [auth] cache_logins/cache_successful_logins_expiry/cache_failed_logins for caching logins
• Add: option [auth] htpasswd_cache to automatic re-read triggered on change (mtime or size) instead reading on each request
• Improve: [auth] log used hash method and result on debug for htpasswd authentication
• Improve: [auth] htpasswd file now read and verified on start
• Improve: [auth] htpasswd: module 'bcrypt' is no longer mandatory in case digest method not used in file
• Improve: [auth] successful/failed login logs now type and whether result was taken from cache
• Improve: [auth] constant execution time for failed logins independent of external backend or by htpasswd used digest method
• Drop: support for Python 3.8

This should

• improvements for Improper fix for timing attacks #591 as constant execution time on failed logins is now implemented and now also covering increased time consumption of external backends like dovecot and ldap implicity
• fix Radicale quite slow after fresh installation with just one item in the calendar using bcrypt authentication #1466 (assume already independent of enabled htpasswd caching)
• support LDAP / AD Account Locking #1639 (if login result cache is enabled)

Unfortunatly, this PR turned bigger and bigger somehow during development :-(

[pbiering]

btw. tests are very welcome!