Tables existence check query is executed in large quantities

When Rails starts, tables existence check query is executed
number of models.
In case of mysql,

    SHOW TABLES LIKE 'table1';
    SHOW TABLES LIKE 'table2';
    SHOW TABLES LIKE 'table3';
    ...
    SHOW TABLES LIKE 'table999';

Add process to get the names of all tables by one query.
Fix spelling.
Merge pull request #16422 from tgxworld/fix_spelling

Closes #16422
change to empty? from blank?
Merge pull request #16421 from tsukasaoishi/prevant_manycall_showtables

Tables existence check query is executed in large quantities
use the uri parser so that newer version of Ruby work
remove dead file_watcher code
Merge pull request #16424 from gregors/remove_dead_code

remove dead file_watcher code
defer running after_config hooks until after the object is allocated
add a new constructor that runs load hooks
need to call super
Stop using padding in the generated Gemfile -- it looks shit
Merge branch 'master' of github.com:rails/rails
Pull spring gem entry into the Gemfile template instead of gemfile_entries so it can be grouped with the other development tools