This updates all the existing memory reference fields to use
`lldb::addr_t` directly.
A few places were using `std::string` and decoding the value in the
request handler and other places had unique ways of parsing addresses.
This unifies all of these references with the `DecodeMemoryReference`
helper in JSONUtils.h.
Additionally, for the types I updated, I tried to simplify the POD types
some and moved default values out of RequestHandlers and into the
protocol POD types.
