add warmup budget to PaddedRSTransport

Adds a 1024 (unpadded) byte sending budget to the PaddedRSTransport below which
messages are instantly flushed down the socket before the transport will
beginn waiting for the buffer to reach MIN_PACKET_SIZE (1024).
This allows to get the first couple of messages sent quickly when
a wallet connects to improve its ux.

Author: f321x

src/electrumx/server/transport.py

@@ -24,6 +24,9 @@ class PaddedRSTransport(RSTransport):
 
     MIN_PACKET_SIZE = 1024
     WAIT_FOR_BUFFER_GROWTH_SECONDS = 1.0
+    # amount of (unpadded) bytes sent instantly before beginning with polling.
+    # This makes the initial handshake where a few small messages are exchanged faster.
+    WARMUP_BUDGET_SIZE = 1024
 
     session: Optional['RPCSessionWithTaskGroup']
 
@@ -57,6 +60,7 @@ def _maybe_consume_sbuffer(self) -> None:
             self._force_send
             or len(buf) >= self.MIN_PACKET_SIZE
             or self._last_send + self.WAIT_FOR_BUFFER_GROWTH_SECONDS < time.monotonic()
+            or self.session.send_size < self.WARMUP_BUDGET_SIZE
         ):
             return
         assert buf[-2:] in (b"}\n", b"]\n"), f"unexpected json-rpc terminator: {buf[-2:]=!r}"