Fix unconditional Return in obfs4 transport

[jmwample]

Fix for unconditional return in obfs4 WrapConnection - move checks on proper data length earlier in the function and continue through loop when the MAC mark that we are looking for is not found.

closes #128
closes #133

Changes

obfs4

• move mark len check proper locations
  • too short check up front
  • not found, but not past max at end for retry
• mark not found w/ one reg, continue to next instead of return
• use tempdir instead of current dir for obfs4 state

PhantomSelector

• update selection to set min and max to correct bounds
• update min and max checks to be inclusive ensuring proper range
  • enables inclusion of short (/32, /31, /128, /127 etc.) subnets
• change ReadVarint to Varint to prevent occasional overflow errors

Proof that the fixes works:

TestSuccessfulWrapMulti - checks that a connection can be established for an obfs4 session even when multiple obfs4 sessions map to the same phantom address.

TestPhantomSeededSelectionFuzz - ensures no errors occur when subnets of all sizes (/0-/32) are used as the phantom subnet for selecting phantom addresses many times over (10,000 selections).

TestPhantomsSeededSelectionV4Min - ensures that minimal subnets work (i.e. /31, /32, /128, etc.)

[jmwample]

The fix(es) applied in this PR relates to #132 as we add a temporary fix that creates an ioutil.tempdir for each obfs4 session ensuring separation of state. However this is probably not a good long-term fix as file descriptor limitations might cause issues, also we probably don't need any of the files to begin with.

[jmwample]

closing in favor of separated solutions: #135 and #136.