Buffered Channels

You can provide a buffer length as the second argument to make() to create a buffered channel:

ch := make(chan int, 100)

A buffer allows the channel to hold a fixed number of values before sending blocks. This means sending on a buffered channel only blocks when the buffer is full, and receiving blocks only when the buffer is empty.

We want to be able to send emails in batches. A writing goroutine will write an entire batch of email messages to a buffered channel, and later, once the channel is full, a reading goroutine will read all of the messages from the channel and send them out to our clients.

Complete the addEmailsToQueue function. It should create a buffered channel with a buffer large enough to store all of the emails it's given. It should then write the emails to the channel in order, and finally return the channel.