Reuse curl handle to attempt to reuse sessions

[cjen1-msft]

Per the mailing list: https://curl.se/mail/lib-2020-01/0015.html
Curl easy handles will try to reuse the SSL sessions etc that already exist.

This PR makes the snapshot fetching use a single curl_easy handle, and thus where possible (no disconnects etc) it should reuse the same session for subsequent connections to the same host.

We shouldn't need to use the CURL_LOCK_DATA_SSL_SESSION as, afaict that applies to multiple easy handles.

[cjen1-msft]

I think from this log we can conclude that it is reusing connections:

*   Trying 127.189.138.158:34139...
* ALPN: curl offers h2,http/1.1
*  CAfile: service_cert.pem
*  CApath: /etc/ssl/certs
* SSL connection using TLSv1.3 / TLS_AES_256_GCM_SHA384 / secp521r1 / id-ecPublicKey
* ALPN: server accepted http/1.1
* Server certificate:
*  subject: CN=CCF Node
*  start date: Sep 30 16:24:21 2025 GMT
*  expire date: Oct  1 16:24:20 2025 GMT
*  subjectAltName: host "127.189.138.158" matched cert's IP address!
*  issuer: CN=CCF Test Service
*  SSL certificate verify ok.
*   Certificate level 0: Public key type EC/secp384r1 (384/192 Bits/secBits), signed using ecdsa-with-SHA384
*   Certificate level 1: Public key type EC/secp384r1 (384/192 Bits/secBits), signed using ecdsa-with-SHA384
* Connected to 127.189.138.158 (127.189.138.158) port 34139
* using HTTP/1.x
> HEAD /node/snapshot HTTP/1.1
Host: 127.189.138.158:34139
Accept: */*

* Request completely sent off
< HTTP/1.1 308 PERMANENT_REDIRECT
< content-length: 0
< location: /node/snapshot/snapshot_46_50.committed
< 
* Connection #0 to host 127.189.138.158 left intact
* Re-using existing connection with host 127.189.138.158
> HEAD /node/snapshot/snapshot_46_50.committed HTTP/1.1
Host: 127.189.138.158:34139
Accept: */*

* Request completely sent off
< HTTP/1.1 200 OK
< accept-ranges: bytes
< content-length: 82516
< 
* Connection #0 to host 127.189.138.158 left intact
* Re-using existing connection with host 127.189.138.158
> GET /node/snapshot/snapshot_46_50.committed HTTP/1.1
Host: 127.189.138.158:34139
Accept: */*
Range: bytes=0-82516

* Request completely sent off
< HTTP/1.1 206 PARTIAL_CONTENT
< accept-ranges: bytes
< content-length: 82516
< content-range: bytes 0-82516/82516
< content-type: application/octet-stream
< 
* Connection #0 to host 127.189.138.158 left intact