Add API to remove expiration for RedisKeyWritable

[QuChen88]

Currently RedisKeyWritable has no easy way to remove expiration/TTL from a key that already exists in Redis because set_expire() takes Duration as input even though the underlying raw::set_expire() method supports this functionality.

This PR adds a new method remove_expire() to RedisKeyWritable.

[QuChen88]

Ping, any updates on this?

[iddm]

Thank you!

[QuChen88]

@MeirShpilraien Can you please take a look? I just rebased the change.

[MeirShpilraien]

@QuChen88 maybe we can add a test for the new functionality?

[QuChen88]

I can't seem to find an existing test for the set_expire() method from before.

What is the convention for adding such a test? Can you point me to an example?

[MeirShpilraien]

@QuChen88 what we usually do is adding a simple example module under the example directory, the module can expose a simple command that uses the new functionality (like a command that remove expiration from a key). Then we add a test that loads the module and check the functionality here: https://github.com/RedisLabsModules/redismodule-rs/blob/master/tests/integration.rs

[MeirShpilraien]

freebds flow seems broken (stuck). Merging without it (tests pass on the other flows).

[MeirShpilraien]

Thanks @QuChen88