[SAMBA CVE-2012-1182] improve bruteforce speed

Improve bruteforce speed

I didn't integrate the "Brute" mixin part as it might be removed in the future if the multithreaded bruteforce  in this module works with all versions (see juan comment @ https://github.com/rapid7/metasploit-framework/pull/987/). This commit is the continuation of work done in PR 987.


Speed comparison : 
- Brute mixin : around 30 tries/min
- this commit : around 100 tries/min

I don't know where the bottleneck is, but I get only around 200/250 KB/s on a gigabyte link. Ruby process runs at 100%.  No "real" threads handling from ruby? GIL being the problem?

Error handling is very rough and would probably need some polishing