reuse ldap connection

[butonic]

@rhafer @longsleep this PR will reuse the global ldap connection and use a dedicated connection for user binds when checking passwords. This limits the amount of memory used when checking user passwords and allows all global requests to reuse the connection, saving the argon2 overhead for the global connection which happens when calling ResolveUserByUsername or GetUser.

fixes https://github.com/owncloud/ocis/issues/5765