Don't use a symbol for the never-cached value

[kuszmaul]

Use an internal singleton value, so that if the user has some reason to make a MultiThreadedCache{K, Symbol} they won't get strange behavior if they happen to store :__MultiThreadedCaches_key_not_found__.

In particular this could happen in the original code:

julia> c=MultiThreadedCache{Int, Symbol}()
MultiThreadedCache{Int64, Symbol}(Dict{Int64, Symbol}())

julia> MultiThreadedCaches.init_cache!(c)
MultiThreadedCache{Int64, Symbol}(Dict{Int64, Symbol}())

julia> get!(c, 4) do; :__MultiThreadedCaches_key_not_found__ end
:__MultiThreadedCaches_key_not_found__

julia> get!(c, 4) do; :hello end
:hello

where we didn't expect to get :hello on the second get!.

[nickrobinson251]

Thanks, @kuszmaul !