GC of solved captchas

[hrj]

• Change solved column to an integer that counts number of times the captcha was solved
• After every N captchas are created, delete the captchas that have been solved more than M times.
• After every N*10 captchas are created, delete the captchas that have been solved more than M/2 times
• Don't serve captchas that have been solved more than M/2 times

N and M need to be configurable, but for now, we can hard-code them to 1000 and 10 respectively.

[rr83019]

Updated design:

• Only one parameter required: M, the limit of times a captcha can be solved. After this limit is reached the captcha will be retired and garbage collected.
• Change solved column to an integer that counts number of times the captcha was solved, initialized to zero.
• Add a timestamp column to the captcha map table, initialized with the current time whenever a captcha is served
• Increment solved column when a captcha is solved
• While selecting a new captcha to be served, the solved count should be less than M
• For GC the solved count should be more than M and the last served time should be less than the current time - time to solve captcha
• For the answer endpoint, it should accept the solution only if the last served time is less than the current time - time to solve
• When the time to solve exceeds or incase of a captcha is garbage collected it should return an error status and ask to request for a new captcha

[hrj]

Closed via #52