Support query execution timeout

[kostja]

Allow to interrupt execution of a complex query when a timeout expires.

[Totktonada]

Why we can not just create a fiber and kill it by timeout?

[Khatskevich]

The first implementation can be the following:

• Set query_timeout variable to query context at the beginning of an execution.
• Check fiber.time64() < query_timeout on each itaration.

Cons:

• do not need any monitoring fiber-killers
• fiber.time64() is fast

Disses:

• fiber.timeout64() is updated on yields only and in case of local memex will not change during all query execution.

[Khatskevich]

Key points:

1. use clock.monotonic64() because it is fast and it changes without yields
2. place the hook to accessor.new(), and move to tgql.compile after Single-point graphql constructor #59