peer: reduce write timeout to 5 seconds
peer: reduce write timeout to 5 seconds

This commit reduces the peer's write timeout to 5s.
Now that the peer catches write timeouts and doesn't
disconnect, this will ensure we spend less time blocking
in the write pool in case others also need to access the
workers concurrently. Slower peers will now only block
for 5s, after every reattempt w/ exponential backoff.

aakselrod/lnd,LightningNetwork/lnd,aakselrod/lnd,LightningNetwork/lnd