Allow calls to IssueLinkManager#createIssueLink and #removeIssueLink to opt-out of indexing

When performing a bulk update of issue links, it may be useful to opt-out of indexing to delay the re-index operation until the operations have been completed.

A use case for this is in GreenHopper's Story -> Epic association code. We have an issue link for a Story to an Epic, and when re-assigning a list of stories to another epic we have for each story:

• Remove the issue link for the existing epic, which re-indexes the story and old epic
• Add the issue link for the new epic, which re-indexes the story and new epic.

This causes two re-indexes on each story, and a re-index on the new epic for each story associated with that epic, as well as potentially multiple re-indexes on the old epic if more than one of the selected issues are currently assigned to the same epic.

(Due to another issue we are forced to re-index the epic another time after this, due to not re-indexing after changing the updated date, which adds even more overhead)

If we could opt out of indexing, we could collect the series of issues that require re-indexing as we go, and perform a single re-index per issue touched.