Added serverStatus fields to IndexBulkBuilder section:

• "spilledRanges"
• "bytesSpilled"

  Description of Linked Ticket

  It would be useful to capture the following diagnostic information in serverStatus about the external sorter:

• Number of sorters in use
• Number of sorter spills
• Amount of data spilled
• Average amount of data written per spill
• Amount of unsorted data buffered in memory
• Amount of data buffered in memory for merging