Improve performance when parsing a very large message from server into llsd

[Dan-Linden]

Related to performance work in #5046. Under some circumstances the viewer will stall for nearly a second.

Debugging shows that viewer is parsing a very large AIS inventory message

Task:
Move parsing (at least for inventory) out of main thread.

Test setup A:

Copy-paste outfits, so you have around 2000. (QA can use account "fancynancy914")

Test Plan:

Clear the viewer cache.
Log in.
Open Fast Timers.
Open outfits list.
Verify the largest stall is less than _____ ?
Relog (needs cache)
Open Fast Timers.
Open outfits list.
Verify the largest stall is less than _____ ?