make the engine reuse its outbound recording consistently

Previously, outbound recordings would get reused in non-deterministic
way after restart, meaning that a recording used by the engine could
become used by the library and the other way round. This is OK, but it's
confusing. Now the engine will reuse its old recording. If there's
a single library, then it will reuse its old recording too.

artio-core/src/main/java/uk/co/real_logic/artio/engine/RecordingCoordinator.java

@@ -288,7 +288,8 @@ public ExclusivePublication trackEngine(final String aeronChannel, final int str
             final boolean isInbound = streamId == configuration.inboundLibraryStream();
             final RecordingIds recordingIds = isInbound ? inboundRecordingIds : outboundRecordingIds;
             final RecordingIdLookup lookup = isInbound ? framerOutboundLookup : framerInboundLookup;
-            final LibraryExtendPosition libraryExtendPosition = acquireRecording(streamId, recordingIds);
+            final LibraryExtendPosition libraryExtendPosition =
+                acquireRecording(streamId, recordingIds, ENGINE_LIBRARY_ID);
             final ExclusivePublication publication;
             if (libraryExtendPosition != null)
             {
@@ -389,16 +390,13 @@ private void extendRecording(
     }
 
     private LibraryExtendPosition acquireRecording(
-        final int streamId, final RecordingIds recordingIds)
+        final int streamId, final RecordingIds recordingIds, final int libraryId)
     {
         libraryExtendPosition = null;
 
-        final Long2LongHashMap.ValueIterator it = recordingIds.free.values().iterator();
-        if (it.hasNext())
+        final long recordingId = recordingIds.acquire(libraryId);
+        if (recordingId != NULL_RECORDING_ID)
         {
-            final long recordingId = it.nextValue();
-            it.remove();
-
             final int count = archive.listRecording(recordingId, this);
             if (count != 1 || null == libraryExtendPosition)
             {
@@ -460,7 +458,7 @@ public LibraryExtendPosition trackLibrary(final int sessionId, final int library
                 }
             }
 
-            extendPosition = acquireRecording(streamId, outboundRecordingIds);
+            extendPosition = acquireRecording(streamId, outboundRecordingIds, libraryId);
             if (extendPosition != null)
             {
                 extendRecording(streamId, extendPosition, extendPosition.newSessionId);
@@ -763,6 +761,23 @@ private void forEach(final Long2LongHashMap map, final LibraryAndRecordingIdCons
             }
         }
 
+        long acquire(final int libraryId)
+        {
+            long recordingId = free.remove(libraryId);
+
+            if (recordingId == NULL_RECORDING_ID)
+            {
+                final Long2LongHashMap.ValueIterator it = free.values().iterator();
+                if (it.hasNext())
+                {
+                    recordingId = it.nextValue();
+                    it.remove();
+                }
+            }
+
+            return recordingId;
+        }
+
         public String toString()
         {
             return "RecordingIds{" +