Make ReviewDb available in the server on a per-thread basis

Allow types bound in the system injector to access the per-thread
ReviewDb if they are invoked from within a request context, without
caring about which entry they were invoked from. This allows email
sending and some other change update action code to be bound at the
global level and work in both SSH and HTTP invocations.

Change-Id: I411ded5196ae6920573233fb0cde48f35ce7ad01
17 files changed