Description:
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While investigating, we found the soap module (SoapClient) is able to request a 
keep-alive HTTP persistent connection (if http/persistent_connection context 
option is set to 1.1, the default):

1/ if you send multiple requests within the same test script, it will correctly 
reuse the connection 
2/ whenever the script ends, connection is released and future calls to the same 
script will end in re-establishing a new HTTP connection (which can be CPU 
intensive when SSL is used)

We found the issue being related to a call to php_stream_xport_create within 
ext/soap/php_http.c where persistent_id parameter is hard coded to be NULL. 

Proposed patch will look at a new stream options named "soap/http_persistent":
- if it set to true, it will fill-in persistent_id param with connection name 
(ex: tcp://host:port or ssl://host:port)
- if it is not set or set to false, persistent_id param remains NULL and actual 
behavior is kept