LDAP util: When checking if attributes are multivalued, store results in a cache.

This prevents us from hitting the schema more often than necessary, and, as a side
effect, causes warnings about attributes not declared in schemata to be printed
only once.
Merge pull request #128 from 4teamwork/lg-ldap-util-cache-multivaluedness

LDAP Util: When checking if attributes are multivalued, store results in a cache