Add md5 checking after downloading a new file handle

This looks like we will require two places to add this check:

1) _sftpDownloadFile
2) _downloadFile (at the end)

The tricky part will be to write a test that is called for every type of download that we support. I.e. when we add support to ftp download (SYNPY-53) we would need to add this check there as well.

All of these functions call _downloadFile:

• _downloadFileEntity

• getWiki (get wiki markdown)

• _downloadWikiAttachment

• _queryTableCsv

• downloadTableFile

• downloadTableColumns

It'd be nice to check MD5 in all cases, but we don't always have a file handle and thus the expected MD5. Getting the filehandle is an option, but is it worth the extra round-trip?

The most important case is downloading a file entity, so, my vote is we leave it at that.

This should address these issues: SYNPY-261, SYNPY-337,

Chris: I don't think that this addresses and as those are related to retrying on failures during download.

I have been using these changes for the last few days and it has caught some errors already.