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out_opensearch: lock the client cache against flush threads (#195)

* out_opensearch: lock the client cache against flush threads

Before: client() cleared the shared @_os slot and then rebuilt it with
`||=`, without a lock. With flush_thread_count of 2 or more and a host
placeholder, a thread could pass the "is this the same host" check and
then pick up the client another thread had just built for a different
host, so its chunk went to that cluster instead.

After: the whole check-and-build runs under a mutex, so a thread always
gets a client for the host it asked for. The mutex is created in
initialize because configure builds a client before it returns.

Co-Authored-By: Claude <noreply@anthropic.com>
Signed-off-by: Kentaro Hayashi <hayashi@clear-code.com>

* out_opensearch: guard the cached client with one mutex

Before:
the credential refresh timer used its own mutex and the flush
thread error path used none, so both could clear @_os while another
thread was building a client under @client_mutex. A failed credential
refresh also threw away the cached client for nothing.

After:
both paths take @client_mutex, and the timer keeps the old
credentials when it cannot get new ones.

Co-Authored-By: Claude <noreply@anthropic.com>
Signed-off-by: Kentaro Hayashi <hayashi@clear-code.com>

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Signed-off-by: Kentaro Hayashi <hayashi@clear-code.com>
Co-authored-by: Claude <noreply@anthropic.com>

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