Add a limit to a number of txns per run of FIX tool

The number of transactions to fix needs to be limited to avoid system overload.

Acceptance criteria:

• Extend the REST API for FIX action and provide a new parameter - maxProcessedTxns
• Add a restriction in AlfrescoCoreAdminHandler#fixOnSpecificCore that will not schedule more than maxProcessedTxns to fix
• The maxProcessedTxns should have a sensible default value configured in solrcore.properties
• The parameter value passed via REST API should take precedence over the value configured in solrcore.properties.