Use readLong/writeLong for serializing seq. no.

When serializing sequence numbers in StartRecoveryRequest we use
readZLong/writeZLong. This is inconsistent with how these are handled in
other places. This commit makes StartRecoveryRequest consistent with the
handling in other places.

Author: jasontedor

core/src/main/java/org/elasticsearch/indices/recovery/StartRecoveryRequest.java

@@ -113,7 +113,7 @@ public void readFrom(StreamInput in) throws IOException {
         metadataSnapshot = new Store.MetadataSnapshot(in);
         primaryRelocation = in.readBoolean();
         if (in.getVersion().onOrAfter(Version.V_6_0_0_alpha1_UNRELEASED)) {
-            startingSeqNo = in.readZLong();
+            startingSeqNo = in.readLong();
         } else {
             startingSeqNo = SequenceNumbersService.UNASSIGNED_SEQ_NO;
         }
@@ -129,7 +129,7 @@ public void writeTo(StreamOutput out) throws IOException {
         metadataSnapshot.writeTo(out);
         out.writeBoolean(primaryRelocation);
         if (out.getVersion().onOrAfter(Version.V_6_0_0_alpha1_UNRELEASED)) {
-            out.writeZLong(startingSeqNo);
+            out.writeLong(startingSeqNo);
         }
     }