stats: SHOW STATISTICS and SHOW HISTOGRAM statements

This issue tracks adding the following statements:
 - `SHOW STATISTICS FOR <table>`: an alias for `SELECT .. FROM system.table_statistics WHERE tableID = ..` The histogram column should be replaced with a boolean column which indicates if there is a histogram.
 - `SHOW HISTOGRAM <statID> FOR <table>`: decodes a histogram and returns it as a table, with one row per bucket.

CC @rytaft 