Fix bug that causes Apisonator to stop pinging System to fetch events

[davidor]

The current code fails if the incr call is made effective in Redis but the client returns an error because of a timeout. When that happens, which should be pretty rare, apisonator stops sending events because from that moment, ping_key_value.to_i == 1 in https://github.com/3scale/apisonator/blob/677dfbf16346bf1efebdf4fec5b5e0a9d72e8ce1/lib/3scale/backend/event_storage.rb#L99 will always be false.

[bors[bot]]

:clock1: Waiting for PR status (Github check) to be set, probably by CI. Bors will automatically try to run when all required PR statuses are set.

[bors[bot]]

Build succeeded:

• ci/circleci: build