Move typedefs to include/Db/Types.hpp

Move the id_type and trid_type typedefs to include/Db/Types.hpp.
Those typedefs were scattered across multiple files before.
Issue #2: Replace Update protobuf message

Replace internal uses of the Update protobuf message by the
Mutation internal data structure.
Fix bug in btree implementation

There was a bug that led to a wrong ordering of
btree nodes when the depth was larger than 2.
Add option to use extern server for unit tests