Prepare included swithes SQLite affinity in pg data type loop

[mkgrgis]

• Prepare C-language infrastructure for ISO:SQL

No SQL behaviour changes. This PR include:

• switches (SQLite data affinity) in switch (PostgreSQL data type)
• add explicit human readable error messages in case of improper combinations of data type and data affinity.

[mkgrgis]

Closed. All contribution moved to https://github.com/pgspider/sqlite_fdw/pull/82