new: add stress_transfer command to test nonce serialization

[vaab]

Summary

Adds a stress_transfer command to exercise the per-account nonce
lock + confirmation under cross-process contention on a test currency.

Fires N back-to-back transferNant transactions to a target
address with no artificial delay. Meant to be run in two shells on the
same host against the same wallet, to validate that concurrent senders
serialize correctly (no nonce gaps or collisions).

Output

For each send it prints:

• the authoritative nonce, re-queried by hash via
  getTransactionInfo (the raw geth tx carries its real nonce);
• the pinned node read from the per-account lock file;
• the tx hash and elapsed time.

At the end it dumps the pending pool across all nodes, filtered to the
wallet address, so queued entries (the smoking gun for a nonce
gap) and collisions are easy to spot.

Usage

# two shells, same host, same wallet, test currency:
pyc3l stress_transfer -w wallet.json -p pass.txt -r A -n 20 -y 0xTARGET
pyc3l stress_transfer -w wallet.json -p pass.txt -r B -n 20 -y 0xTARGET
# add -l pyc3l:INFO to see the lock contention logs

Note

This exercises the nonce-serialization fix in pyc3l
(com-chain/pyc3l#5).