Update wsrpc for improved ping handling

Add error codes to server-sent messages

These error codes can provide more actionable information to a client
about the failed state of their mix, instead of simply being
disconnected from the server for an unknown reason and the client
seeing an EOF.

Currently two codes are defined: one for a catch-all server abort of
the session for an unspecified reason, and a second indicating that
the unmixed data submitted by the client (in a coinjoin, the tx
containing inputs and change outputs) is invalid.

Move SR/DC messages to Session, generate fresh X25519 keys

The SR/DC messages may be reused across runs, but a new run structure
is created for each run.  Moving these messages to the Session type
allow them to be reused correctly.

The X25519 keys must be uniquely created for each run after secrets
are revealed.  Move the X25519 key generation to the same freshgen
check as SR/DC message generation.

Double client read timeouts

An issue was observed where a wallet successfully signed a mix and
responded to the server, but failed to read the resulting transaction
with all combined signatures.  It's suspected that this wallet was
faster to sign and respond to the mix, causing it to set an earlier
deadline than the server was able to respond within.  Doubling the
recv timeout should help.