Replace protoc generated SerializedBlock with DynamicMessage

BinaryBlock format depends on a protobuf (SerializedBlock). This protobuf classes are generated during the build using protoc configured at runtime (default version 2.4.1). Since protobuf 2.4 and 2.4 are incompatible, this causes a runtime error when used with protobuf 2.5.

Since SerializedBlock is a fairly simple protobuf, this patch uses DynamicMessage built at runtime. This avoids any dependence on pre-generated protobuf code. This should make EB work with both protobuf 2.4.1 and 2.5