MySQL BLOB support and MySQL 3.23 build

Hello!

I have two patches:

1. Patch for enable SOCI compilation with MySQL 3.23 (Note that MySQL 3.23.
    It consist in removing the "CLIENT_MULTI_RESULTS" macro if it is not defined.
    It applies to "src/backends/mysql/session.cpp".

2. Initial BLOB support for the MySQL backend.
    I did it several months ago. It is based on the "sqlite3" backend code. I need help to test it. It applies to "src/backends/mysql/blob.cpp".

This is my first "Pull Request" :-). I am sorry if I made any inconvenience.