New read request doesn't abort old requests.

[MendyBerger]

Seems like selecting a new set of words to read, the extension doesn't abort the old requests, if the first hasn't had a chance to start yet.

Steps to reproduce:

• I select text to read.
• Nothing happens.
• I select a different set of words to read.
• Starts reading second selection.
• In the middle of reading second selection, it starts reading the first selection.

This issue has started to occur recently.