When deciding whether a collation or character set change should prohibit
using the inplace algorithm for an ALTER TABLE statement, the collation
handlers were compared, and if they were the same, the change was
accepcted even for indexed fields.

However, we can use the same collation handler for different collations,
so the decision should instead be based on comparing the pair of character
set and collation being used for the source and destination field.

Additionally, changing the collation should be prohibited for ALTER inplace
if the column is indexed in the target table. Thus, changing the collation
and adding a new index on the same column should be prohibited for ALTER
inplace, while changing the collation and dropping an existing index
should be allowed.

Change-Id: I2d0c7fb2161103d4d05d29be0f62bde69a4aed0d