Make sure options are propagated to child Writes

[jonjohnsonjr]

Right now we are just passing through cherrypicked options from
WriteIndex to other WriteIndex and Write calls. This isn't sustainable
as we add more options, since we'd have to keep these in sync.

Instead, for WriteIndex, we can reuse the writer by pulling the
implementation out into writer.writeIndex.

For Write, since we do scope calculation based on layers, we can't reuse
the writer because we don't do scope calculation for WriteIndex.

[codecov-io]

Codecov Report

Merging #818 (e237685) into master (6a63025) will decrease coverage by 0.06%.
The diff coverage is 53.57%.

@@            Coverage Diff             @@
##           master     #818      +/-   ##
==========================================
- Coverage   74.77%   74.71%   -0.07%     
==========================================
  Files         103      103              
  Lines        4313     4314       +1     
==========================================
- Hits         3225     3223       -2     
- Misses        611      614       +3     
  Partials      477      477              

Impacted Files	Coverage Δ
pkg/v1/remote/multi_write.go	57.54% <0.00%> (ø)
pkg/v1/remote/write.go	65.09% <55.55%> (+0.12%)	⬆️
pkg/v1/remote/options.go	61.53% <0.00%> (-7.70%)	⬇️

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