Parse Net::LDAP::PDU when calling Net::LDAP::Connection#read, instrument PDU parsing

[mtodd]

This PR reduces the duplication by moving the PDU parsing into the Net::LDAP::Connection#read method and instruments PDU parsing to give more visibility into the messages being sent from the server.

Should not result in any behavior change.

I believe this is along similar lines to #116, though not exactly the same.

For what it's worth, I looked at instrumenting the Net::LDAP::PDU.new method from the inside but there's no reference to the instrumentation_service object to service the instrumentation events (and I didn't want to go the route of a global default handler). This seemed like a good balance and a needed refactoring anyway.

Done:

• parse PDU in Net::LDAP::Connection#read
• instrument parse_pdu.net_ldap_connection event
• unit tests
• cleanup/refactoring of Net::LDAP::Connection#read callsites

Still to do:

• integration tests (optional)

Many of the read callsites look like this:

(pdu = read) or raise Net::LDAP::LdapError, "no bind result"

This should be cleaned up, and this PR is a good opportunity to do so. Done.

cc @jch @schaary

[jch]

👍 🚢

I would've preferred separate pull requests for the #read refactoring and instrumentation, but it's a good set of changes. I also like the additional integration tests (but those could've also been a separate PR as well 😉)

[mtodd]

@jch yeah, gonna try to make sure subsequent PRs are more focused.