Async ILdapUserStore

[scattered-code]

Since LDAP calls are I/O bound, async calls could add a performance benefit under high-load.

[chrgraefe]

Hi,
the underlying library (Novell.Directory.Ldap.NETStandard) isn't supporting async methods yet.
There is development effort to implement this.
see here:
issue: dsbenghe/Novell.Directory.Ldap.NETStandard#27
branch (develop): https://github.com/dsbenghe/Novell.Directory.Ldap.NETStandard/tree/develop

please stay tuned

[Nordes]

I've upgraded the lib today. However, I don't know if they did anything about it.

new branch described in #65 (comment)