Re-introduce LDAP user caching

EnterpriseLdapManager in NX2 used to provide user-level caching to avoid repeated requests for the same information from going upstream to the LDAP server. This was necessary to avoid a flood of upstream LDAP requests from token-based realms such as NuGet or Usertoken.

When the LDAP codebase was ported to NX3 this caching appears to have been removed from EnterpriseLdapManager. This story will look at re-introducing user-level caching to the NX3 LDAP codebase, so all token-based realms (and other users of LDAP-based information) can take advantage of this cache and don't have to implement their own caching.

See https://issues.sonatype.org/browse/NEXUS-13467?focusedCommentId=415462&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-415462 for more background.