Single-copy strings

[dktapps]

It recently came to my attention that it's possible to make only a single copy of strings, if the assigning context keeps the assigned string around long enough to be copied directly.

This would involve:

• Assigning thread puts the string into the thread-local Threaded object's cache and adds a ref to it. It also adds a pointer to this string to the shared storage.
• When the Threaded object which cached the string is destroyed, it must copy the string to shared storage before deallocating itself (this ensures that, for example, copying a string from a dead child thread to parent works correctly).
• Strings must be cleaned from Threaded local cache in the same way that Threaded, Closure and Socket objects currently are.

This would give us a performance improvement in PM in many cases:

• Copying packets from main -> RakLib would now only involve 1 copy, on the RakLib thread - reducing main thread time spent on sending packets
• Copying packets from RakLib -> main thread would only involve 1 copy, on the main thread - reducing RakLib time spent on receiving packets
• Async compression would involve only 2 copies instead of 4: AsyncTask would have to copy uncompressed from main, and main would have to copy compressed from AsyncTask (instead of both having to copy both ways).

[dktapps]

A problem that might nerf the advantage of this is needing to do a bytewise comparison of strings to determine whether the cached version is the same as the local version.