Eliminate as much thread-local state as possible

[nafg]

Perhaps use scala-query's pattern: pass around implicitly where possible, or provide an opt-in global implicit for thread-local state.

[nafg]

After a great conversation with Alexey Romanov, it's become clear that thread-local is never correct. The new architecture should look like this:
Javascript is sent to a Page.
There can be many possible transport mechanisms, with various characteristics.
Page tracks the "currently available" transports (plural) and chooses the most efficient.

[nafg]

Fixed on branch v0.4.0 in commit 18b49c6