GC of solved captchas about lc-core HOT 2 CLOSED

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

from lc-core.

Closed via #52

from lc-core.