[RFR] 8160768: Add capability to custom resolve host/domain names within the default JDNI LDAP provider

[Rob McKenna]

This recently received CSR approval, so it seems like a good time to pick
the codereview up again:

http://cr.openjdk.java.net/~robm/8160768/webrev.08/

Referencing:

http://mail.openjdk.java.net/pipermail/core-libs-dev/2017-December/050794.html

1) I'm copying the behaviour from
LdapCtxFactory.getInitialContext(Hashtable<?,?>) where an empty String is
the default value used when the provider url is null.

I don't think HostnameChecker (by way of SNIHostname) will accept either
empty or null values when doing the comparison.

Somewhat oddly however, LdapCtx.extendedOperation(ExtendedRequest)
appears to pass the String "null" to
StartTlsResponseImpl.setConnection(Connection, String), which attempts
to substitute null values with the Connections host so there may be a bug
here.

I'm happy to allow null returns here if necessary. Sean, can you
comment on the distinction between null & "" as far as hostname
verification is concerned?

2) In the latest iteration lookupEndpoints() returns an
Optional<LdapDnsResult>. Currently neither getEndpoints() nor
getDomainName() can be null. (they can be an empty list and/or an empty
String however)

3) Corrected.

4) See https://www.oracle.com/technetwork/java/seccodeguide-139067.html#4-5

    -Rob