SDK 0.4.31: connection reuse in the synchronous client

The synchronous client keeps HTTP/1.1 connections for reuse instead of opening a new TCP and TLS connection for every request, and builds its TLS context once. A connection returns to the pool only after its response body is fully read. Idle connections retire after 5 seconds, as in the async client, and a connection the peer has closed is discarded before reuse. Only GET and HEAD are resent after a reused connection fails. Proxied requests still use urllib.

On Hetzner, a sequential exec from a laptop took 81 ms instead of 196 ms, and client CPU for 50 concurrent execs fell from 17 s to 0.6 s. No backend version change is required.