WiredTiger: In-memory databases

| Version 3.1.0

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In-memory databases

The wiredtiger_open `in_memory` configuration changes WiredTiger to run in cache without writing to a backing disk. Data is limited to the configured cache size.

If `in_memory` is configured, WT_CURSOR::insert and WT_CURSOR::update methods may return an additional error, WT_CACHE_FULL, indicating the insert or update operation requires more than the configured cache size to complete. If a transaction is in progress, it should be rolled back.