Support for simultaneous LDAP and Kerberos authentication.

Prior to this work, the impalad could either authenticate with
Kerberos, or authenticate with LDAP.  This fixes that so that both can
co-exist in the same daemon.  Prior code had both a
KerberosAuthProvider and an LdapAuthProvider; this is refactored into
a single SaslAuthProvider that potentially contains both LDAP and
Kerberos.

The terminology of "client facing" and "server facing" has been
replaced with "external" and "internal".  External is for clients like
the impala shell, odbc, jdbc, etc.  Internal is for daemon <-> daemon
communication.

The notion of the "auxprop" plugin is removed, as that was dead code.

The Thrift code is enhanced to pass the Realm information from the
SaslAuthProvider down to the underlying SASL library.

Change-Id: I0a0b968a107c0b25610ca37295c3fee345ecdd6d
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4051
Reviewed-by: Michael Yoder <myoder@cloudera.com>
Tested-by: jenkins