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This change adds support for multiple concurrent requests, as well as
loading multiple models by spawning multiple runners. The default
settings are currently set at 1 concurrent request per model and only 1
loaded model at a time, but these can be adjusted by setting
OLLAMA_NUM_PARALLEL and OLLAMA_MAX_LOADED_MODELS.

Cleaner shutdown logic, a bit of response hardening

If images aren't present, pull them.
Also fixes the expected responses

This uplevels the integration tests to run the server which can allow
testing an existing server, or a remote server.