Provide bulk method to removeAttachments in AttachmentManager

Issue Summary

This is reproducible on Data Center: (yes) 

A feature request for moveAttachments API so that only few Synchrony reconciliations are done for each content only after all attachments are moved

Steps to Reproduce

1. Currently each attachment move triggers one content reconciliation on the container of the attachment.
2. In case of large number of attachment moves, it can overwhelm the system with 2-3 times the transactions.

Expected Results

Only 2-3 reconciliation requests are created after all attachment moves.

Actual Results

Reconciliation requests in 2-3 times the number of attachments are raised at the moment.

Notes

The problem can only be reproduced when Collaborative Editing is enabled.

Workaround

Currently there is no known workaround for this behavior. A workaround will be added here when available

Impact on apps

ConfiForms
This has an impact on the ConfiForms - Data Forms & Workflows app causing the form submission (in case of a "Move Attachment" IFTTT action) to fail with a message similar to the following:

WARN [http-nio-8090-exec-3001 url: /ajax/confiforms/trysave.action; user: dbbadmin] [confluence.util.profiling.DurationThresholdWarningTimingHelperFactory] logMessage Execution time for post-commit task com.atlassian.confluence.internal.content.collab.AttachmentRelatedContentReconciliationListener$$Lambda$4088/0x0000000803ddf440@4673afda took 176513 ms (warning threshold is 5000 ms)

Possible option to address the problem:

• instead of using the current ConfiForms definition, create a new page. This will reset the need for the reconciliation logic to follow a significant number of content/attachment links.