Implement this interface for the Envelope handler for R2DBC Projections.
It can be stateful, with variables and mutable data structures. It is invoked by the Projection machinery one
envelope at a time and visibility guarantees between the invocations are handled automatically, i.e. no volatile or
other concurrency primitives are needed for managing the state.
Supported error handling strategies for when processing an Envelope fails can be defined in configuration or using
the withRecoveryStrategy method of a Projection implementation.