Adding IV support to SYM_ENCRYPT and ASYM_ENCRYPT protocols (JGRP-2386).

[nsawadsky]

This change allows JGroups to support symmetric encryption algorithms that require an initialization vector (IV). An important example is "AES/CBC/PKCS5Padding" which uses the CBC cipher mode.

The main changes that were required are as follows:

• Added a sym_iv_length field to Encrypt. This must be set to a non-zero value when using a cipher mode that requires an IV.
• Added an iv field to the EncryptHeader (which may be null if the cipher mode does not require an IV).
• Since the IV is passed when the Cipher is initialized and the IV is created per-message, cipher initialization is now deferred until the message is being encrypted/decrypted.
• Since the key is passed to the Cipher.init() method and init() is deferred, we now store the Key rather than the Cipher in the key_map field of Encrypt.
• DH_KEY_EXCHANGE is mostly unchanged. So it does not support using an IV-based cipher to encrypt the shared symmetric key. This seems okay as a fresh DH session key is generated for each key exchange, so the ciphertext will always be different. This saves us having to add an iv field to the DhHeader.
• All the SYM_ENCRYPT and ASYM_ENCRYPT tests have been extended with subclasses that run the same tests using "AES/CBC/PKCS5Padding".

[belaban]

LGTM. The 'encrypt' task passed fine, now running the entire testsuite... stay tuned.

[belaban]

@nsawadsky Do you think a small addition to the manual might make sense?

[belaban]

@nsawadsky Thanks for your contribution!

[nsawadsky]

Thanks Bela! I will open another pull request for the manual updates.