Helperify Massdns

[TheTechromancer]

This PR promotes massdns to a scan-level helper that can be accessed like this:

self.helpers.dns.brute(module, domain, subdomains)

This PR also contains various code organization improvements:

• dnscommonsrv has been updated to use the new brute helper
• the (very huge and complex) massdns module has been split into two smaller, simpler ones: dnsbrute and dnsbrute_mutations
• new bloom filter implementation to allow for larger brute-force runs with lower memory requirements (dnsbrute_mutations memory usage is now capped at 100MB)

TODO:

• Update module names, config options in documentation

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 93.00971% with 36 lines in your changes are missing coverage. Please review.

Project coverage is 92%. Comparing base (1720e42) to head (b202086).
Report is 3 commits behind head on bbot-2.0.

Files	Patch %	Lines
bbot/modules/dnsbrute_mutations.py	88%	12 Missing ⚠️
bbot/core/helpers/dns/brute.py	89%	11 Missing ⚠️
bbot/modules/base.py	0%	4 Missing ⚠️
bbot/core/helpers/bloom.py	96%	2 Missing ⚠️
...t/test_step_2/module_tests/test_module_dnsbrute.py	97%	2 Missing ⚠️
...p_2/module_tests/test_module_dnsbrute_mutations.py	92%	2 Missing ⚠️
...st_step_2/module_tests/test_module_dnscommonsrv.py	85%	2 Missing ⚠️
bbot/modules/templates/subdomain_enum.py	92%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           bbot-2.0   #1303    +/-   ##
=========================================
+ Coverage        92%     92%    +1%     
=========================================
  Files           328     333     +5     
  Lines         22067   22224   +157     
=========================================
+ Hits          20124   20314   +190     
+ Misses         1943    1910    -33     

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