Meta cache breakdown

[melange396]

1 SQL query per distinct (src, sig) instead of 1 global

[melange396]

so in sum total, the applying this change to the db would either require wiping/recreating/repopulating from scratch, or applying this SQL:

ALTER TABLE `covidcast` RENAME COLUMN `timestamp1` to `value_updated_timestamp`;
ALTER TABLE `covidcast` RENAME COLUMN `timestamp2` to `direction_updated_timestamp`;

ALTER TABLE `covidcast` ADD `is_wip` BINARY(1) DEFAULT (NULL);

-- and then either these (should be faster)
UPDATE TABLE `covidcast` SET `is_wip` = 1 WHERE `signal` LIKE 'wip_%';
UPDATE TABLE `covidcast` SET `is_wip` = 0 WHERE `is_wip` IS NULL;

-- or these (at least slightly safer)
UPDATE TABLE `covidcast` SET `is_wip` = 0 WHERE `is_wip` IS NULL and `signal` NOT LIKE 'wip_%';
UPDATE TABLE `covidcast` SET `is_wip` = 1 WHERE `is_wip` IS NULL and `signal` LIKE 'wip_%';

[korlaxxalrok]

@melange396 The necessary changes should be done.

[AlSaeed]

Merge Conflict Resolution PR:
melange396#1

[AlSaeed]

LGTM