bump time to first completion up to 3.5 seconds to allow for more syntactically complete code completions

[beyang]

It's a better user experience to wait a bit longer to generate a syntactically whole completion rather than a completion that does not complete the syntactic block. The issue with the latter is both the sense of "incompleteness" but also, we get "stuck" in a more deeply nested block.

Column 1	Column 2

Test plan

Tested locally