Fix for 429393 - org.eclipse.birt.data.oda.pojo.impl.internal.ClassMethodFieldBuffer is not threadsafe

Issue happens because of race condition on shared
ClassMethodFieldBuffer. 

Rework on ClassMethodFieldBuffer.
In this fix, each connection will has its own instance of
ClassMethodFieldBuffer.

Signed-off-by: xinzhao-acutate <xzhao@actuate.com>