Protect wallet data file with a file lock

Operations on the wallet data file are now fenced by a lock to
avoid potentially messy concurrent modifications by multiple
processes (i.e. the wallet receiver and a send command). The lock
is done using a create-only lock file, which is an atomic
operation.

pool/src/pool.rs

@@ -114,7 +114,6 @@ impl<T> TransactionPool<T> where T: BlockChain {
         // Making sure the transaction is valid before anything else.
         let secp = secp::Secp256k1::with_caps(secp::ContextFlag::Commit);
         tx.validate(&secp).map_err(|_| PoolError::Invalid)?;
-        
 
         // The first check invovles ensuring that an identical transaction is 
         // not already in the pool's transaction set.