Patch: add recovery_timeout option to control timeout of restore_command nonzero status code

Hello everyone.

* Project name:  Add recovery_timeout option to control timeout of restore_command nonzero status code
* Uniquely identifiable file name, so we can tell difference between your v1 and v24:  0001-add-recovery_timeout-to-controll-timeout-between-res.patch
* What the patch does in a short paragraph: This patch should add option recovery_timeout, which help to control timeout of restore_command nonzero status code. Right now default value is 5 seconds. This is useful, if I using for restore of wal logs some external storage (like AWS S3) and no matter what the slave database will lag behind the master. The problem, what for each request to AWS S3 need to pay, what is why for N nodes, which try to get next wal log each 5 seconds will be bigger price, than for example each 30 seconds. Before I do this in this way: " if ! (/usr/local/bin/envdir /etc/wal-e.d/env /usr/local/bin/wal-e wal-fetch "%f" "%p"); then sleep 60; fi ". But in this case restart/stop database slower.
* Whether the patch is for discussion or for application: No such thing.
* Which branch the patch is against: master branch
* Whether it compiles and tests successfully, so we know nothing obvious is broken: compiled and pass tests on local mashine.
* Whether it contains any platform-specific items and if so, has it been tested on other platforms: hope, no.
* Confirm that the patch includes regression tests to check the new feature actually works as described: No it doesn't have test. I don't found ho to testing new config variables.
* Include documentation: added.
* Describe the effect your patch has on performance, if any: shouldn't effect on database performance.
This is my first patch. I am not sure about name of option. Maybe it should called "recovery_nonzero_timeout".

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Alexey Vasiliev