Disable unsused features from flume dependency

[domenicquirl]

This PR modifies the dependency on flume to only enable its async feature instead of all features that are enabled by default. The motivation for this change is that neither rumqttc nor rumqttd need to select between (i.e., wait for any one of multiple) messages from multiple channels. However, the select feature of flume is enabled by default, together with its eventual-fairness feature that ensures that channels in a selector cannot make other ones starve. To do so, a source of randomness is required which is obtained through additional dependencies that this PR aims to avoid for both users and developers of rumqtt.

Type of change

Miscellaneous (related to maintanance)

Checklist:

• Formatted with cargo fmt (only Cargo.tomls are changed)
• Make an entry to CHANGELOG.md if its relevant of user of the library. If its not relevant mention why.

[domenicquirl]

Is a CHANGELOG entry required for this change? The change should be transparent for any users, except if they were unknowingly relying on rumqtt enabling the features while also depending on flume without specifying them.

Based on the previous entries it looks to me like this wouldn't appear in the list, but I can include a mention if you prefer.

[h3nill]

no changelog entry not required for this

[h3nill]

Thank you @domenicquirl!