it prepares the new architecture change,
everything about multithreading in moved in the searx.search.* packages
previously the call to the "init" function of the engines was done in searx.engines:
* the network was not set (request not sent using the defined proxy)
* it requires to monkey patch the code to avoid HTTP requests during the tests
The new url parameter "timeout_limit" set timeout limit defined in second.
Example "timeout_limit=1.5" means the timeout limit is 1.5 seconds.
In addition, the query can start with <[number] to set the timeout limit.
For number between 0 and 99, the unit is the second :
Example: "<30 searx" means the timeout limit is 3 seconds
For number above 100, the unit is the millisecond:
Example: "<850 searx" means the timeout is 850 milliseconds.
In addition, there is a new optional setting: outgoing.max_request_timeout.
If not set, the user timeout can't go above searx configuration (as before: the max timeout of selected engine for a query).
If the value is set, the user can set a timeout between 0 and max_request_timeout using
<[number] or timeout_limit query parameter.
Related to #1077
Updated version of PR #1413 from @isj-privacore
