http2: only schedule write when necessary

[addaleax]

• src: add optional keep-alive object to SetImmediate

  Adds the possibility to keep a strong persistent reference to
  a JS object while a SetImmediate() call is in effect.

• http2: don't call into JS from GC

  Calling into JS land from GC is not allowed, so delay
  the resolution of pending pings when a session is destroyed.

• http2: only schedule write when necessary

  Introduce an Http2Scope class that, when it goes out of scope,
  checks whether a write to the network is desired by nghttp2.
  If that is the case, schedule a write using SetImmediate()
  rather than a custom per-session libuv handle.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

http2