md5 hash the hostname in the native extension

[hudon]

The native extension implementation for next_object_id doesn't match Ruby's. The latter hashes the hostname while the former does not. This patch fixes the native extension to also MD5 hash the hostname.

[amacneil]

For context (and seo for others running into this issue) this bug has been causing a large number of duplicate ID errors for us:

The operation: #<Moped::Protocol::Command @length=81 @request_id=351 @response_to=0 @op_code=2004 @flags=[] @full_collection_name="db.$cmd" @skip=0 @limit=-1 @selector={:getlasterror=>1, :w=>1} @fields=nil> failed with error 11000: "E11000 duplicate key error index: db.collection.$_id_ dup key: { : ObjectId('55ba4acc69702d000100016a') }" See https://github.com/mongodb/mongo/blob/master/docs/errors.md for details about this error.

Investigating further, the hostname portion of the ObjectId from all servers was set to ip-:

> BSON::ObjectId.from_string('55ba4acc69702d000100016a').to_bson[4..6]
=> "ip-"

This happened to be the prefix of all our machines' hostnames, which meant that if any two machines generated an ID in the same second with the same counter, we got an ObjectId clash.

[durran]

Thanks. I'll get a release out quickly.

[durran]

3.2.1 is out with this fix - thanks again.