ipc/stream: add idle timer to vc/vifs

 https://github.com/veyron/release-issues/issues/847

 As described in this issue, this CL
   * Closes VC if there is no active user flow for X duration, and
   * Closes VIF as soon as all VCs created on it have been closed.

 The default timeout is 30 minutes for now.

Change-Id: Ibe5f4a22e566cfdd1c13b1d9eea28f352976c052
23 files changed