add shutdown command

[SeanChinJunKai]

Closes #1425

[SeanChinJunKai]

@arpitbbhayani, @JyotinderSingh and @lucifercr07 I am not too sure how to implement SHUTDOWN command. I have added the command file. However, I think that once SHUTDOWN command input we should be calling cancel function from server/main.go however that will require passing the cancel function as a parameter.

[arpitbbhayani]

@SeanChinJunKai yes. we will need to pass the ctx through and through. that is how we should gracefully shutdown the database.

[SeanChinJunKai]

@arpitbbhayani I shall figure something out

[arpitbbhayani]

@SeanChinJunKai any updates? are you still working on this?

[SeanChinJunKai]

@arpitbbhayani sorry i’ve been quite busy and haven’t had time to work on it

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