Make the lock object static readonly

There exists a race conditions where a thread may try to modify the
static MapperRegistry.Mappers collection while another is enumerating
the collection in the LINQ query. There is a lock statement, but I dont
think its catching it if multiple instance of
PlatformSpecificMapperRegistryOverride are instantiated by
ProbingAdapterResolver.

Making the lock object static readonly should alleviate this

Fixes #639