Support for thread neutral operation of mapserver/mapscript

[szekerest]

The purpose of this change is to get a significant performance boost by removing
"big locks" from the code. It would add a significal improvement of the
applications having high number of multiple threads executed simultaneously.
This activity could bring in a more feasible support for some application models
not really aimed at now like Microsoft ASP.NET. May result in higher clarity of
the code by determining the code segments affected by multiple threads.

This bug will collect the developer and user comment according to this change.
The proposal is collected into:

http://mapserver.gis.umn.edu/development/rfc/ms-rfc-15/