LDAP connector: implement connection pooling

[ericchiang]

The LDAP connector implemented in #178 creates a new LDAP session for every request, closing and discarding the connection after login.

Add connection reuse to the existing logic. After attempting to login, connections should be:

• "reset" by binding anonymously
• kept alive by occasionally pinging the LDAP server (or discarded if this fails)
• reused on future login attempts by other dex end-users

[philips]

@ericchiang @fnordahl can one of you flesh the description out further?

[fnordahl]

@philips I do not have access to edit the description of this issue, but I have fleshed out the description for the PR. Cc @ericchiang

[ericchiang]

👍 updated here

[fnordahl]

thx!