Optimize SQL Queries for image counting and statistics

[jeawhanlee]

Is your feature request related to a problem? Please describe.
Slow queries can some times cause time out on affected pages. see here

Describe the solution you'd like
Optimize the SQL Queries for the image counting and statistics

• imagify-plugin/inc/functions/admin-stats.php

  Line 12 in e046686

  function imagify_count_attachments() {

• imagify-plugin/inc/functions/admin-stats.php

  Line 70 in e046686

  function imagify_count_error_attachments() {

• imagify-plugin/inc/functions/admin-stats.php

  Line 121 in e046686

  function imagify_count_optimized_attachments() {

• imagify-plugin/inc/functions/admin-stats.php

  Line 233 in e046686

  function imagify_count_saving_data( $key = '' ) {

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.