SqlMapper AddTypeMap/RemoveTypeMap not thread safe

AddTypeMap:
https://github.com/StackExchange/Dapper/blob/master/Dapper/SqlMapper.cs#L230

RemoveTypeMap:
https://github.com/StackExchange/Dapper/blob/master/Dapper/SqlMapper.cs#L244

Both methods make snapshots of the type map before they update it, however they do not ensure the reference has not been modified before they replace it (as might be implemented with Interlocked.CompareExchange)
This can cause one thread to overwrite another's change.

Eg Thread A calls AddTypeMap(XType, DbType) and Thread B calls AddTypeMap(YType, DbType), one possible execution path is:

A takes a reference to the current typeMap and verifies it does not contain XType
B takes a reference to the current typeMap and verifies it does not contain YType
A creates a copy of typeMap
B creates a copy of typeMap
A inserts XType into its copy of typeMap
A sets the real typeMap variable to use its copy
B inserts YType into its copy of typeMap
B sets the real typeMap variable to use its copy <- B's copy does not contain XType, so it effectively erases A's changes here.

Start in Dapper/SqlMapper.cs at AddTypeMap around line 230 and RemoveTypeMap around line 244. Trace how each method snapshots and replaces the type map, then verify concurrent updates do not overwrite one another. Done means changes from simultaneous AddTypeMap or RemoveTypeMap calls are all preserved.