Fix missing write deadline in writeRLPx

[damilolaedwards]

Summary

writeRLPx locked writeMu and wrote to the rlpx connection with no write deadline. A peer that completed the handshake and then stopped reading could stall the write indefinitely while the lock was held, wedging the session and every other sender behind it, since they all serialize on the same mutex.

Sets a write deadline before the write, mirroring the read deadline already applied each iteration of the session loop. The timeout is a package variable rather than a constant so tests can shrink it instead of waiting out the full duration.

Test plan

• Added a test that drives a real handshake against an unresponsive peer and confirms the write now fails within the deadline instead of blocking forever.
• Verified the test fails against the previous code (a goroutine-local reference to the accepted connection let the GC finalizer close the socket early during an earlier draft, producing a misleading result; fixed by having the connection owned by a long-lived test peer struct instead) and passes with the fix.
• go build ./..., go vet ./..., and the full package test suite all pass.