Fix bug

Forgot about TTL value
Add schema as parameter to BluefloodEndpoint
Update TODO for BluefloodEndpoint
Fail plugin init when no mandatory config keys are present

Currently, there are two such keys: URL, Tenant
Add test for wrong type/data during write
Test flush fail due to connection reasons

Exceptions are tolerated. They don't break anything, just increase
next `write` call time. Moreover, they are quite informative.