MINIFICPP-1117 - minifi::Exception is now nothrow copyable

[szaszm]

Implemented in terms of std::runtime_error

Thank you for submitting a contribution to Apache NiFi - MiNiFi C++.

In order to streamline the review of the contribution we ask you
to ensure the following steps have been taken:

For all changes:

• Is there a JIRA ticket associated with this PR? Is it referenced
  in the commit message?

• Does your PR title start with MINIFICPP-XXXX where XXXX is the JIRA number you are trying to resolve? Pay particular attention to the hyphen "-" character.

• Has your PR been rebased against the latest commit within the target branch (typically master)?

• Is your initial contribution a single, squashed commit?

For code changes:

• If adding new dependencies to the code, are these dependencies licensed in a way that is compatible for inclusion under ASF 2.0?
• If applicable, have you updated the LICENSE file?
• If applicable, have you updated the NOTICE file?

For documentation related changes:

• Have you ensured that format looks appropriate for the output in which it is rendered?

Note:

Please ensure that once the PR is submitted, you check travis-ci for build issues and submit an update to your PR as soon as possible.

[szaszm]

I've added some boilerplate for string joining. I can remove it at some performance cost (extra std::string allocations) or additional dependency (boost::algorithm::join)

[szaszm]

update:

• fix ProcessorTests
• move string utilities to StringUtils and generalize them
• add unit test to cover the new StringUtils::join_pack

Moving the utilities from a private namespace to StringUtils means that they need to be more general IMO. I've extended them to work with any character type and do SFINAE checking on the string types.

This means quite some template code, which may not be welcome in this codebase. Suggestions are welcome.

[arpadboda]

LGTM