[BUG] `GET /channels/{channelId}/statistics` returns the counters of an unrelated channels

[mgaffigan]

Repro:

1. In one thread, loop: deploy channel B, send a message through it, undeploy channel B.
2. In another thread, loop: GET /channels/{A}/statistics

Expected:

• GET /channels/{A}/statistics always succeeds and gives the statistics for channel A

Actual:

• While B is mid-deploy or mid-undeploy, the response describes channel B.