schemadiff: fix TableCharsetCollateIgnoreAlways and TableCharsetCollateIgnoreEmpty and normalized column charset & collation

[shlomi-noach]

Comparing tables with TableCharsetCollateStrategy = TableCharsetCollateIgnoreAlways or TableCharsetCollateIgnoreEmpty can generate a wrong plan depending on column definitions. As example: take these from and to statements:

from: "create table t (a varchar(64)) default charset=latin1",
to:   "create table t (a varchar(64) CHARACTER SET latin1 COLLATE latin1_bin)",

In the from definition, a's charset is really latin1, derived from the table. So is the charset in to, defined explicitly.

However, when converting between the two tables in TableCharsetCollateIgnoreAlways, the fact the table charset is ignored leads to the wrong behavior where the column's charset is modified.

PR is incoming with test cases and fixes.