SPIKE: Namadillo: Tx processing in the background

[jurevans]

We should investigate using a worker to handle Tx build calls in the background (especially for MASP transfers).

• Should background building happen in a Service Worker, where the lifespan of the service worker can survive past that of a tab? Or instead just use a Web Worker or Shared Worker?
• Perhaps we could sync notifications across tabs (pending Tx - issue once Tx is building in SW), but only broadcast on a single tab: If the user closes the window, and build Tx continues in the background, we could potentially retrieve it to broadcast, but that needs some thought
• RE: SPIKE: Multithreading in WASM #1059 - a multithreading SDK wasm should still behave the same way if a MASP tx build is issued from within the SW

NOTE We could make services generic enough that we could also do background processing of IBC transfers, where any IBC tx in progress can later update when applied to destination chain.