[FIX] Validate coin_address on account updates

Fixes #945 once merged.
Merge pull request #946 from TheSerapher/issue-945

[FIX] Validate coin_address on account updates
[IMPROVED] Use limits for DELETE statements

This will address #886. Long rounds can cause a system to become
very unresponsive due to high SQL/IO load when doing cleanups of
shares and archived tables.

* Run DELETE from shares with LIMIT
* Run DELETE from shares_archive with LIMIT
* Configure DELETE behaviour via config file
* Only archive shares that are really required (PROP, PPS)

Should greatly improve round ends on PROP and PPS after large rounds,
also improves PPLNS though archving will still take some time unless
we limit the share amount artificially. Shares could be needed though,
so we don't.
[CHANGE] Adjusted purge defaults
[REMOVED] Old unused stats on mobile
Merge pull request #952 from TheSerapher/issue-949

[REMOVED] Old unused stats on mobile
Merge pull request #890 from TheSerapher/issue-886

[IMPROVED] Use limits for DELETE statements
Fixed assignment bug in findblock cron
[UPDATE] Increase login input cap
Merge pull request #974 from Huri/next

Fixed assignment bug in findblock cron
[UPDATE] Added indexes for share archive
Merge branch 'next' of https://github.com/TheSerapher/php-mpos into next