Add option for lazy-loading fields

[nickwilliams-eventbrite]

The JIRA class constructor provides several fields to prevent (or enable) initially connecting to JIRA to get information from it. For example, get_server_info will enable connecting to get server information if True, but disable it if False.

However, the end of the constructor has this code:

        self._fields = {}
        for f in self.fields():
            if 'clauseNames' in f:
                for name in f['clauseNames']:
                    self._fields[name] = f['id']

This makes it impossible to construct a JIRA object without connecting to JIRA, even if you have set the 2-3 "check things" arguments to False. In a test or development environment, it's common to have dummy/do-nothing configurations for such things, and stub out or patch the code doing the actual connection/calling, but when you can't even construct an object without connecting, it makes testing more difficult. Not impossible, mind you, just more difficult.

Additionally, self._fields is, currently, only used while searching, so connecting and retrieving this information eagerly wastes I/O if you aren't going to use the JIRA instance for searching.

I propose making self._fields be a lazy-loading property, instead. It would be compatible with all code currently using self._fields, but it would delay connecting and retrieving this field information util necessary.

    @property
    def _fields(self):
        if not hasattr(self, '_fields_value'):
            self._fields_value = {}
            for f in self.fields():
                if 'clauseNames' in f:
                    for name in f['clauseNames']:
                        self._fields_value[name] = f['id']
        return self._fields_value

[stale]

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[nickwilliams-eventbrite]

Stalebot, please remove the stale label. An issue does not become stale just because it's not acknowledged/replied to.