OPENDJ-2813 Tolerate malformed attribute descriptions when filtering entries

LDAP RFC 4511 specifies that a search operation's attribute list should
comprise of either "*", "+", or a valid attribute description. There is
also an extension which allows clients to specify object classes using
"@" prefixes.

Following the principle of being tolerant when receiving protocol, this
change relaxes the parsing of attribute descriptions such that invalid
attribute descriptions are simply ignored.