Now, the checksum field is a CHAR(32) instead of a BIGINT.
Previously it was failing on some MySQL versions since the truncated int
it was storing is not consistent. Now, by having an MD5 stored as a
CHAR(32) the checksum is consistent across MySQL and OS versions.
This is a breaking change and pt-query-digest history and review tables
are not backwards compatible.
In different OSes, using BIGINT to store the checksum for a query, will
produce collisions and/or unstable values.
Because of this, I changed the type for the checksum field in the history
table from BIGINT to CHAR(32) and the checksum function is no longer
truncating the MD5_HEX result to 16 chars.
