[keyserver] added target_message to messages table

Summary:
added target_message to messages table. target_message is the id of the message that is being reacted to. Also added the index to target_message column and this is to optimize how we will be fetching reaction messages for a certain message

Linear Task: ENG-2265

Test Plan:
Please see the screenshot to see that target_message is added to the messages table

Also dropped and created my local DB to test the setup works as expected. Tested the stack locally in my environment to make sure everything still ran as expected/nothing crashed.

Reviewers: atul, tomek, rohan, ashoat

Reviewed By: atul, ashoat

Subscribers: ashoat, tomek, atul, abosh

Differential Revision: https://phab.comm.dev/D5658