Backport most important bug fixes

• Resolves: upstream#3523 - ABRT crash - /usr/libexec/sssd/sssd_nss in setnetgrent_result_timeout
• Resolves: upstream#3562 - Use-after free if more sudo requests run and one of them fails, causing a fail-over to a next server
• Resolves: upstream#3588 - sssd_nss consumes more memory until restarted or machine swaps
• Resolves: failure in glibc tests https://sourceware.org/bugzilla/show_bug.cgi?id=22530
• Resolves: upstream#3451 - When sssd is configured with id_provider proxy and auth_provider ldap, login fails if the LDAP server is not allowing anonymous binds
• Resolves: upstream#3285 - SSSD needs restart after incorrect clock is corrected with AD
• Resolves: upstream#3586 - Give a more detailed debug and system-log message if krb5_init_context() failed
• Resolves: #1479283 - proxy to files does not work with implicit_files_domain
• Resolves: #1431153 - SSSD ships a drop-in configuration snippet in /etc/systemd/system