Clean up headers with include-what-you-use

[mloskot]

There is lots of places where #include can be potentially replaced with forward declarations. For example, soci-mysql.h does not need #include <mysql.h>, neither soci-postgresql.h needs #include <libpq-fe.h>.

This could loose coupling, shorten compilation times, remove what's unused.

Here is fantastic tool that can do the job: http://code.google.com/p/include-what-you-use/

[Gui13]

+1, especially since soci-mysql uses #include <mysql.h> whereas some installations put the header into <mysql/mysql.h>