merge MMcM: Sizes of MCOMPAT_ BLOB and MEDIUMBLOB were backwards. Likewise TEXT and MEDIUMTEXT.

Merge pull request #1116 from MMcM/sql-layer/types3-medium-lob-sizes
https://github.com/FoundationDB/sql-layer/pull/1116
BINARY : VARBINARY :: CHAR : VARCHAR
Add *exactly* compatible size computations via TInstance.