db support for treasury data

[chappjc]

Next step to support a new treasury page is new PG queries or even a new treasury table to fill out a treasury page with the three types of txns:

• treasuryadd burns coins and credits the treasury account.
• treasuryspend creates coins and debits the treasury account.
• treasurybase is analogous to a coinbase in that it credits the treasury without spending/burning any coins, instead creating them out of thin air like coinbases

collect and store todo here: https://github.com/decred/dcrdata/pull/1817/files#diff-1256fb31f67b81d11af39cef8f5f93723821756f546f2b939b77d96280d5615aR3942

Also, the votes with a 4th output are used to authorize treasury spends so we should consider how to track these types of votes and/or their tspend decisions (decoded via stake. GetSSGenTreasuryVotes or stake.CheckSSGenVotes). Related is the gettreasuryspendvotes RPC that can tally the yes/no votes on a particular tspend.

Originally posted by @chappjc in #1817 (comment)