Improve socialcast-net-ldap mirroring

Multiple users experience issues during their mirroring process when run against certain versions and/or configurations of Microsoft's Active Directory. The error message observed is: Error Searching: invalid response-type in search: 24. Response type 24 in the NET::LDAP gem indicates an unsolicited notification [notice of disconnection]. Basically after reaching message ID 128 on their LDAP connection, Net::LDAP returns Protocol errors.

Discussion in staging about this issue:
https://er.staging.socialcast.com/messages/1187779

Another party resolved this by switching from net-ldap to ruby-ldap: 
http://stackoverflow.com/questions/9621930/net-ldap-gem-in-ruby-invalid-response-type-in-search-24

This has been reproduced internally on Active Directory (AD) on Windows Server 2008 R2 64Bit. After populating the directory with 40000 users, the mirror task was run and it was confirmed that a msgid incremented past 127 gets misinterpreted. While some AD servers doesn't reject this, the target AD server of larger customers appear to have more strict behavior in relation to the expected msgid [http://www.ietf.org/rfc/rfc2251.txt - section 4.1.1], which produces the fatal error mentioned above.

this gem modification avoids using the msgid range 128-255 by starting the msgid counter at 300.

A similar issue observed with this gem:
http://www.openldap.org/lists/openldap-software/200007/msg00130.html