ARROW-2799: [Python] Add safe option to Table.from_pandas to avoid unsafe casts

[kszucs]

Depends on both #2497 and #2503

[kszucs]

There is a spurious but pertinacious conda error:

CondaHTTPError: HTTP 500 INTERNAL SERVER ERROR for url <https://conda.anaconda.org/conda-forge/linux-64/repodata.json>
Elapsed: 00:00.413982
CF-RAY: 4541d1fc0b3e54f8-ORD
A remote server error occurred when trying to retrieve this URL.
A 500-type error (e.g. 500, 501, 502, 503, etc.) indicates the server failed to
fulfill a valid request.  The problem may be spurious, and will resolve itself if you
try your request again.  If the problem persists, consider notifying the maintainer
of the remote server.

[wesm]

This happens quite often. Is there a way to configure conda to retry some number of times, or wait and retry?

[xhochy]

The 500 errors were a general outage of conda which was resolved. conda in the newest version should do some retries.

[xhochy]

@kszucs The dependencies were merged. Can you rebase?

[kszucs]

@xhochy rebased, CI fails because mesonbuild

[xhochy]

+1, LGTM

[wesm]

The parquet-cpp merge has caused our merge script to do something pretty wacky. I'm going to try to figure it out

[wesm]

Rebased