Remove support for DCE Security UUID (V2) generation

[theckman]

This change removes our support for generating V2 UUIDs from the package, since
it seems that the current implementation is not up to spec in relation to the
DCE 1.1 Specification[1][2].

In our implementation the data written in to the V1, V3-V5 UUID uses Big Endian
byte order as specified in RFC-4122[3]. However, when looking at the DCE
specification they make it read like the data should be written in not Little
Endian but a format where you completely reverse the bites of the value from
most significant first to least significant first:

Set the time_low field equal to the least significant 32-bits (bits numbered
0 to 31 inclusive) of the time stamp in the same order of significance. If a
DCE Security version UUID is being created, then replace the time_low field
with the local user security attribute as defined by the DCE: Security
Services specification.

There are more things later confirming the order:

Set the time_mid field equal to the bits numbered 32 to 47 inclusive of the
time stamp in the same order of significance.

This timestamp handling instructions seem dangerous because the specification is
telling us to overwrite the lest-significant bits of the UUID timestamp value
with a deterministic value, meaning the remaining bits of the timestamp will now
change much less frequently. Because the rest of the UUID consists of distinct /
deterministic values, this will make the UUID much less unique.

This also means that for the DCE Security UUIDs we would need completely
different generating and parsing code than V1 and V3-V5 UUIDs. Considering that
it seems like there would be quite a bit of work to support them correctly.

To confirm that my understanding of the spec is correct, I tried to look at
preexisting implementations to validate my understanding. Unfortunately, I've
not had luck finding a UUID library that implements V2 UUIDs. That points to
there being little to no value in continuing to try and support them, as there
is not widespread availability of complementing implementations.

This change will require a major version bump.

[1] http://pubs.opengroup.org/onlinepubs/9629399/apdxa.htm#tagcjh_20_02_05
[2] http://pubs.opengroup.org/onlinepubs/9668899/chap5.htm#tagcjh_08_02_01_01
[3] https://tools.ietf.org/html/rfc4122#section-4.1.2

Signed-off-by: Tim Heckman t@heckman.io

[codecov-io]

Codecov Report

Merging #71 into master will decrease coverage by 0.04%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #71      +/-   ##
=======================================
- Coverage   99.05%   99%   -0.05%     
=======================================
  Files           4     4              
  Lines         318   303      -15     
=======================================
- Hits          315   300      -15     
  Misses          2     2              
  Partials        1     1

Impacted Files	Coverage Δ
uuid.go	100% <ø> (ø)	⬆️
generator.go	96.77% <ø> (-0.45%)	⬇️

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[adamdecaf]

There look to be some callers, but it sounds like they're broken anyway?

https://github.com/search?q=language%3Ago+%22uuid.NewV2%22&type=Code

[niaow]

We should probably remove the info about v2 UUIDs at the top of uuid.go.

Other than these issues, it seems good.

[niaow]

LGTM