GH470: Synchronize access to a shared KeyPairGenerator, which may not be thread-safe.

The keyPairGenerator object in the MontgomeryCurve is a bouncycastle implementation
of the java.security.KeyPairGenerator class. The generateKeyPair method in class
org.bouncycastle.jcajce.provider.asymmetric.edec.KeyPairGeneratorSpi is not thread safe,
so calling the keyPairGenerator.generateKeyPair method must be synchronized.

Author: zakharovsergey1000

sshd-core/src/main/java/org/apache/sshd/common/kex/MontgomeryCurve.java

@@ -153,7 +153,9 @@ public Digest createDigest() {
     }
 
     public KeyPair generateKeyPair() {
-        return keyPairGenerator.generateKeyPair();
+        synchronized (this) {
+            return keyPairGenerator.generateKeyPair();
+        }
     }
 
     public byte[] encode(PublicKey key) throws InvalidKeyException {