Avoid passing mutable collections in a multi-threaded context.
The accumulation data was stored in a mutable Set, and when something
called 'getAndClear...' on it, it passed the set directly into the
callback. This ended up leading to lost data, since the set could be
cleared before the callback acted on it. I changed things to use a
SetBuilder, and passing the result to the callback before clearing the
builder. Yay immutability!
More specific CSS selector for the 'processing upload data' overlay.
To avoid having the 'Processing upload data' text show up on any other spinner overlays that happen to be on the page (i.e. the trace ending indicator)