Use deferred to be full async in retry

[joelwurtz]

Q	A
Bug fix?	yes
New feature?	no
BC breaks?	no
Deprecations?	no
License	MIT

What's in this PR?

It adds a deferred class which allow to resolve retry in an async way (no more wait call)

Why?

Previously using retry plugin would force async to be sync

Checklist

• Need to updated CHANGELOG.md (wait for other PR to be merged to not have useless conflicts)

[dbu]

this can now be rebased and made ready for merging ;-)

[joelwurtz]

It should be good now.

However this plugin may still need a warning, as we have a call to the sleep function inside which is synchronous, there is the possibility to have asynchronous sleep but it heavily depends on the implementation (amp / react / ...), so there is no possibility to add it here

[dbu]

However this plugin may still need a warning, as we have a call to the sleep function inside which is synchronous

do you want to add something for this topic to the documentation of the retry plugin?

[joelwurtz]

Done in php-http/documentation#237