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

[mogita]

Problem

The SDK's calls are synchronous execute() calls, which Dispatcher.maxRequestsPerHost does not limit, so concurrency was never capped at 5. But ConnectionPool(5, ...) kept only 5 idle connections, so after a burst every other connection was closed and the next burst redid the TCP and TLS handshakes.

Solution

• Raise DEFAULT_MAX_CONNS_PER_HOST from 5 to 100, so up to 100 connections stay warm for reuse. Idle connections still close after 55s.
• Correct the comment that said the Dispatcher caps in-flight requests: it only caps async enqueue() calls.
• 100 matches stream-py (#299) and the other SDKs.

How to verify

1. ./gradlew test --tests 'io.getstream.StreamHTTPClientTest'
2. testDefaultPoolKeepsBurstConnectionsWarm runs 10 concurrent requests against a local server and asserts that all 10 connections stay idle in the pool. With the old default it fails with expected: <10> but was: <5>.