Use pnode->nLastRecv as sync score directly

NodeSyncScore() should find the node which we recv data most recently, so put a negative sign to pnode->nLastRecv is indeed wrong.

Also change the return value type to int64_t.

Signed-off-by: Huang Le <4tarhl@gmail.com>
Rebased-By: Wladimir J. van der Laan <laanwj@gmail.com>
Rebased-From: 09a54a6