Refactor BackendMessageSubscriber and Conversation.

Encapsulate access to nested sink and demand fields with methods. Replace Atomic* fields with volatile ones that are updated through AtomicLongFieldUpdater. Use Operators.addCap(…) to accumulate demand and to prevent overflow.
Introduce buffer for received Conversation that do not have demand. Introduce drainloop to drain the buffer on request(n). Extract constants. Use Operators.on…Dropped(…) to propagate dropped signals.

[resolves #231]

Author: mp911de

src/main/java/io/r2dbc/postgresql/client/Client.java

@@ -66,10 +66,10 @@ public interface Client {
     Mono<Void> close();
 
     /**
-     * Perform an exchange of messages.
+     * Perform an exchange of messages. Note that the {@link ReadyForQuery} frame is not emitted through the resulting {@link Flux}.
      *
      * @param requests the publisher of outbound messages
-     * @return a {@link Flux} of incoming messages that ends with the end of the frame (i.e. reception of a {@link ReadyForQuery} message.
+     * @return a {@link Flux} of incoming messages that ends with the end of conversation (i.e. reception of a {@link ReadyForQuery} message. Th
      * @throws IllegalArgumentException if {@code requests} is {@code null}
      */
     default Flux<BackendMessage> exchange(Publisher<FrontendMessage> requests) {
@@ -79,8 +79,10 @@ default Flux<BackendMessage> exchange(Publisher<FrontendMessage> requests) {
     /**
      * Perform an exchange of messages.
      *
-     * @param requests the publisher of outbound messages
-     * @return a {@link Flux} of incoming messages that ends with the end of the frame (i.e. reception of a {@link ReadyForQuery} message.
+     * @param takeUntil the predicate that signals the resulting {@link Flux} to terminate. Typically a check if the {@link BackendMessage} is the last frame of a conversation. Note that the
+     *                  {@link BackendMessage} that matches the predicate is not emitted through the resulting {@link Flux}.
+     * @param requests  the publisher of outbound messages
+     * @return a {@link Flux} of incoming messages that ends with the end of conversation matching {@code takeUntil}. (i.e. reception of a {@link ReadyForQuery} message.
      * @throws IllegalArgumentException if {@code requests} is {@code null}
      * @since 0.9
      */