NAKACK: overwriteDigest() doesn't adjust sequence number

NAKACK.overwriteDigest() is used by the 2 state transfer protocols (STATE_TRANSFER and STREAMING_STATE_TRANSFER).

When A has a digest A:250,B:10,C:160, and gets the state (as a result of calling JChannel.getState() with a digest of A:230,B:10,C:160, it'll set its own digest to A:230 (from A:250).

However, A's sequence number (seqno) will remain at 250 ! This means, when A sends the next message, it'll send A:251, but because we set the digest to A:230, A will ask itself to retransmit messages A:230-250 !

SOLUTION: when receiving digest A:230, A should set its seqno to 230
WORKAROUND: use FLUSH, which doesn't ship digests on state transfer