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ARROW-4080: [Rust] Improving lengthy build times in Appveyor

This tries to cut the build times by skipping:

1. build for stable (it doesn't seem too useful).
1. benchmarks in travis
2. build for dev profiles in windows CI - now we only build with release
   profiles.

Author: Chao Sun <sunchao@apache.org>

Closes #3231 from sunchao/ARROW-4080 and squashes the following commits:

f5956404 <Chao Sun> Disable some flaky doctests
60f8b7d2 <Chao Sun> ARROW-4080:  Improving lengthy build times in Appveyor