Avoid releasing sent buffer to early in BookieClient mock

If the buffer was sent to more than one bookie with the mock, it would
be released after being sent to the first one. Each write should
retain a refCount themselves, and then release when done.

Author: Ivan Kelly <ivank@apache.org>

Reviewers: Sijie Guo <sijie@apache.org>

This closes apache#1598 from ivankelly/double-rel-mock

bookkeeper-server/src/test/java/org/apache/bookkeeper/client/MockBookKeeperTestCase.java

@@ -511,13 +511,15 @@ protected void setupBookieClientAddEntry() {
             boolean isRecoveryAdd =
                 ((short) options & BookieProtocol.FLAG_RECOVERY_ADD) == BookieProtocol.FLAG_RECOVERY_ADD;
 
+            toSend.retain();
             executor.executeOrdered(ledgerId, () -> {
                 byte[] entry;
                 try {
                     entry = extractEntryPayload(ledgerId, entryId, toSend);
                 } catch (BKDigestMatchException e) {
                     callback.writeComplete(Code.DigestMatchException,
                             ledgerId, entryId, bookieSocketAddress, ctx);
+                    toSend.release();
                     return;
                 }
                 boolean fenced = fencedLedgers.contains(ledgerId);
@@ -528,6 +530,7 @@ protected void setupBookieClientAddEntry() {
                     if (failedBookies.contains(bookieSocketAddress)) {
                         callback.writeComplete(NoBookieAvailableException,
                                 ledgerId, entryId, bookieSocketAddress, ctx);
+                        toSend.release();
                         return;
                     }
                     if (getMockLedgerContentsInBookie(ledgerId, bookieSocketAddress).isEmpty()) {
@@ -538,6 +541,7 @@ protected void setupBookieClientAddEntry() {
                     callback.writeComplete(BKException.Code.OK,
                             ledgerId, entryId, bookieSocketAddress, ctx);
                 }
+                toSend.release();
             });
 
             return null;