Cache Dialect when creating a session factory

[maca88]

Fixes: #1950

This is a quick fix for the issue as a proper solution would probably require to refactor the logic for creating a session factory.

[fredericDelaporte]

This fix the session factory building case, but maybe there are other cases which may suffer of re-creating the dialect.
I was thinking about doing something more wide but also a bit more hackish: an internal lazy property on Mapping, but reinitialized on some Configuration method calls like SecondPassCompile.

[maca88]

I've tried to move the Dialect property from IMapping to ISessionFactoryImplementor and the only usage that was problematic is the one from the issue. The IMapping implementation in SessionFactoryImpl uses the dialect from the Settings class that has it stored in a property. The only recreation that I additionally found can happen at:

nhibernate-core/src/NHibernate/Cfg/Configuration.cs

Line 1709 in ce73fc0

var result = settingsFactory.BuildSettings(GetDerivedProperties());

Where both BuildSettings and GetDerivedProperties use the Dialect.GetDialect method. As those methods are called only once per session factory I don't think that they are problematic.

[fredericDelaporte]

Back-ported to 5.2.4.