p2p: update frame size (#235)

Reflect the change made in https://github.com/tendermint/tendermint/pull/5805

The MTU (Maximum Transmission Unit) for Ethernet is 1500 bytes.
The IP header and the TCP header take up 20 bytes each at least (unless
optional header fields are used) and thus the max for (non-Jumbo frame)
Ethernet is 1500 - 20 -20 = 1460
Source: https://stackoverflow.com/a/3074427/820520

spec/p2p/peer.md

@@ -51,7 +51,7 @@ It goes as follows:
     - get 64 bytes of output from hkdf-sha256
     - if we had the smaller ephemeral pubkey, use the first 32 bytes for the key for receiving, the second 32 bytes for sending; else the opposite.
 - use a separate nonce for receiving and sending. Both nonces start at 0, and should support the full 96 bit nonce range
-- all communications from now on are encrypted in 1024 byte frames,
+- all communications from now on are encrypted in 1400 byte frames (plus encoding overhead),
   using the respective secret and nonce. Each nonce is incremented by one after each use.
 - we now have an encrypted channel, but still need to authenticate
 - extract a 32 bytes challenge from merlin transcript with the label "SECRET_CONNECTION_MAC"