sampler: Support thereafter of zero

[abhinav]

The Zap sampling logger accepts three configuration parameters:
interval, first, and thereafter.

After we see first log with the same message in interval,
the sampler kicks in.
Following that, we let through every thereafter-th log.

So for example,

NewSamplerWithOptions(core, time.Second, 100, 50)

This will allow 100 logs with the same message in a second,
and following that, every 50th message with that message.

In #1032, the user wanted the sampler to reject all duplicate messages
once the limit was reached, but our sampler panics if thereafter = 0.

This change adds support for setting thereafter to 0,
dropping all messages in that interval once the limit is reached.
It also adds further explanation to the documentation to address the
misuse in #1032.

Resolves #1032

[codecov]

Codecov Report

Merging #1033 (65e213f) into master (10b1fe4) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1033   +/-   ##
=======================================
  Coverage   98.20%   98.20%           
=======================================
  Files          47       47           
  Lines        2067     2067           
=======================================
  Hits         2030     2030           
  Misses         29       29           
  Partials        8        8           

Impacted Files	Coverage Δ
zapcore/sampler.go	100.00% <100.00%> (ø)

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[sywhang]

[mholt]

Thank you! The updated docs and support for M=0 are much clearer to me.