[folbricht]

The SFTP and SSH stores opened all N connections when the store was created, whether they were needed or not. With a high -n that meant starting that many SSH processes up front. This is also a prerequisite for adaptive concurrency (#430), where the number of requests in flight changes during a run and the connections should follow it.

Both stores now share a small pool (sessionpool.go) that:

• opens the first connection when the store is created, so a wrong host or path still fails at startup
• opens further connections only when all existing ones are busy, up to N, and blocks callers once N are in use
• opens connections one at a time, since SSH may fail when several processes use the SSH agent concurrently
• waits for all connections to be returned on Close, and reports every close error instead of only the last one

A failure to open a later connection is now returned by the request that needed it rather than when the store is created. A pool size below 1 is treated as 1; previously a store created with N: 0 blocked forever on its first request.

The S3 store kept minio's default of 16 idle connections per host, so with a concurrency above 16 the extra connections were closed and reopened, with a new TLS handshake, for every request. It now keeps one per concurrent request, as the HTTP and OCI stores already do. Raising the per-host limit alone isn't enough, since minio also caps idle connections at 256 in total, so that cap is raised too. The OCI store had the same problem above 100, the total cap of the Go default transport it clones, and gets the same fix. GCS already keeps 100 per host and is unchanged.