F# 4.3.1.0 and concurrency fixes

Added an F# 4.3.1.0 specific RProviderServer to deal with serialization
issues caused by conflicting FSharp.Core assemblies between the
VisualStudio process and the RProviderServer process. Also, added a lock
around access to the server object in RServer info to prevent multiple
RProviderServer process creation. Finally, added a lock around access to
the R engine in RProviderServer to eliminate the occasional error
encountered when there are multiple threads accessing the engine at the
same time.