`TotalHotkeyAlpha` is severely bloated

[bdmason]

At block 5175760 TotalHotkeyAlpha has 128252 entries, out of those 91835 (71.6%) have zero balances and 36417 have non zero balances.

It makes querying this storage item (for all entries, which is necessary in a few indexing scenarios) over 3 times slower than it needs to be, as well as bloating chain nodes.

The numbers for TotalHotkeyShares are similar (though I'd expect them to be the same):

Total: 128084
Zeros: 91835
Non-Zeros: 36249 <- a few less