Add support for fractional seconds timeout

[mvorisek]

• usleep supports microseconds resolution
• Redis support milliseconds expire times

The changed method prototypes imply minor BC break if overriden, but no BC break in general, as float does accept (original) int values implicitly even with strict types.

• composer.json format is fixed using https://github.com/ergebnis/composer-normalize

• callable return types are templated to infer the real return type

[willemstuursma]

Hi, thank you for this MR. I was a on a holiday, but I'll have a look!

[willemstuursma]

Had a quick look, looks pretty fine, I'll have a closer look later this week.

Thank you, a very nice contribution!

[mvorisek]

@willemstuursma may I please ask to review this PR?