perf: raise the default connection pool size from 5 to 100

[mogita]

Problem

The default SocketsHttpHandler capped each client at 5 connections per server (MaxConnectionsPerServer = 5). The API is served over HTTP/1.1, so a 6th concurrent request queues for a free connection, and that wait counts against the request timeout.

Solution

• Raise the MaxConnsPerHost default from 5 to 100 in StreamOptions and ClientBuilder. Idle connections still close after 55s.
• 100 matches stream-py (#299).
• Apps that relied on the old limit to throttle traffic can set MaxConnsPerHost = 5.

How to verify

1. dotnet test tests --filter "FullyQualifiedName~ConnectionPoolTests|FullyQualifiedName~ClientBuilderTests"
2. All 20 pass and assert MaxConnectionsPerServer and the client.initialized log line at 100.