There was no way to atomically check for a key in the table,
and then setting if it if non existant. Back before persistent
this was not a problem, however now it is, as its possible for
values to be added before team's robot programs start. This makes
the old method of calling Put*** methods in RobotInit invalid.
This adds SetDefault methods, which do this atomically.
Updates are merged or themselves updated as user code changes. This avoids
the need for the dispatcher to do this and also avoids the need for disabling
updates when the dispatcher isn't running, because there's no risk of memory
usage piling up.
Previously, setters were locally but not globally atomic because they
used GetEntry() (globally atomic) in conjunction with locally atomic
gets/sets to the StorageEntry. To support synchronizing network handshakes
they need to be globally atomic.
GetEntry() has been removed due to this issue, so a helper was added to
StorageTest instead.
