SERVER-9444 Use Sorter in DocumentSourceSort

For now external sorting is disabled, but this lays the groundwork to
enable it.
SERVER-9444 improve Sorter serialization of Document and Value

This is needed for correct handling of missing values.
SERVER-9444 Remove context-awareness from Accumulators to prep $group for Sorter
SERVER-9444 Make $group use Sorter when results don't fit in memory
Remove dead classes: DocMemMonitor and StringWriter
Extra safety to prevent Sorter from writing to disk in mongos

Follow-up to SERVER-9444
Enable test for SERVER-4534

It was fixed when index building switched to using the new Sorter.
Make Sorter file numbers globally unique within a process.

Previously they were only unique per included file.
Add user-facing option for SERVER-9444: Using disk storage for $sort and $group