OGR support is not threadsafe in MapServer

[matthew@…]

When using MapServer in a multi-threaded application (such as an ASP.NET
application) ORG is not thread safe during operations that access the same data. 

MapServer seems to be a lot "more" thread safe with shapefiles, which leads me
to believe that the issue is with OGR.

[fwarmerdam]

Generally speaking OGR is not considered thread safe, and there are some 
very serious and well known thread safety issues with the GetFieldAsString()
method on the OGRFeature. 

There needs to be a "big lock" around OGR in MapServer similar to the one
already implemented around GDAL. 

I'll try and implement this before MapServer 5.0 release. 

[szekerest]

> Generally speaking OGR is not considered thread safe, and there are some 
> very serious and well known thread safety issues with the GetFieldAsString()
> method on the OGRFeature.

This issue has been fixed when GDAL RFC 6 was applied. But I'm sure it was not
the only problem to be handled.

> There needs to be a "big lock" around OGR in MapServer similar to the one
> already implemented around GDAL.

It should be considered as a makeshift for now, and the usage of the static
variables in OGR should be reconsidered in the future.

[warmerdam]

I have introduced TLOCK_OGR and added use of it in mapogr.cpp in trunk (r6656 and r6657).

Proceeding to testing phase. I will also be working with Tamas on C# test app for multi-threaded rendering.

[warmerdam]

I have done some validation with a client provided test suite and things seem good.

Closing.