add a wal-check function in wal-e to check if a segment exists on s3.

Summary:
as the title, check if single segment exists

Test Plan: run it.
envdir /etc/wal-e.d/env /usr/local/bin/wal-e wal-check pg_xlog/0000000600004C02000000B8

returncode is 0 if exists.

Reviewers: lguo

CC:

Task ID: #

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