With this change, doUpload method will retry the multipart upload on NoSuchUploadError if the "retry-on-no-such-upload" flag is used and the remote file has not been modified after the beginning of the execution (or does not exists at all) . Otherwise (if it has been modified) it will assume that upload was successfull, and ignore the error.

Beware that, this workaround approach has some drawbacks to consider:
1) If upload really failed but a third person succesfuly modified the remote file (dsturl), then this code will incorrectly return "success".
	2) It would be better to use checksum rather than the last modification date but, unfortunately it is not supported in aws-sdk-go v1.*, and it is a bit tricky to use ETag's as a replacement of checksum for the multipart uploads. Refer https://docs.aws.amazon.com/AmazonS3/latest/userguide/checking-object-integrity.html#large-object-checksums
	 see also the following as an example of using ETag as checksum https://github.com/peak/s3hash

Updates #450

Write simple test.

The tests does not check if the logic works as expected for when the varios last modification dates of remote. All the test assumes that there is no (such) object in remote (since the response is cleared & replaced with NoSuchUpload error).

Remove unused "StartTime" variable.
Change flag name to  "no-such-upload-retry-count" from "retry-on-no-such-upload". Also change corresponding fields' name to [N]oSuchUploadRetryCount.
Move code generation to an external method.

... even though it is a hidden flag.

It will use debug level to log retry attemp.  Debug level is also the level. that s3.customRetryer write logs, see also
https://github.com/peak/s5cmd/blob/822043839c0e581f9b481c9ef30dd88a04463a09/storage/s3.go#L932-L934