register rtdb triggers per-namespace in functions emulator

[jmwski]

Description

In the original commit used to wire together the functions emulator with the database emulator, we did not register function triggers per-namespace. Without a namespace query paramter (ns), the RTDB emulator assumes the request is for a special fake sentinel namespace called 'fake-server'. This is confusing because developers may expect that the function emulator registers triggers with a namespace whose name matches the project id of their currently active firebase project.

In the current state of things, writes to a namespace whose name matches the active firebase project id would not trigger RTDB functions because those triggers are always registered with the sentinel namespace.

Scenarios Tested

I tested triggering an RTDB function for a specified namespace, something like:

curl -X PUT -d '{"data": true}' http://localhost:9000/rtdbReact.json?ns=my-namespace

Sample Commands

[coveralls]

Coverage decreased (-0.002%) to 61.367% when pulling 39b1d3a on wyszynski/rtdb-emulator-per-namespace-triggers into 589387b on master.

[samtstern]

@IanWyszynski gonna merge. No need for a changelog since that will be included in the changelog for my RTDB change.