Improve documentation of performance_schema tables by appending COLUMN
comments to tables. Additionally improve test coverage and update corresponding
tests.
Before this fix, the performance schema tables were defined in UPPERCASE.
This was incompatible with the lowercase_table_names option, and caused
issues with the install / upgrade process, when changing the lower case
table names setting *after* the install or upgrade.
With this fix, all performance schema tables are exposed with lowercase names.
As a result, the name of the performance schema table is always lowercase,
no matter how / if / when the lowercase_table_names setting if changed.
