Before this fix, the performance schema table 
events_statements_summary_by_digest was a summary grouped
by column DIGEST alone.

With this fix, a new column SCHEMA_NAME is added,
and the digest summary is grouped by columns SCHEMA_NAME and DIGEST.

The fix consist of:
- changing the LF_HASH key used for digests to include the schema name,
- adjusting all the code using the digest hash accordingly.
- adding a new SCHEMA_NAME column,
- adjusting all the code accordingly,
- adjusting all tests accordingly.