Coin selection memory exhaustion

[braydonf]

Versions
bcoin 0551096 (May 31st, 2019)

Expected

For coins to be selected via a database query as to not load all coins into memory.

Actual

Coins are selected by loading all coins for an account and sorted random, age or by value. For an account or wallet with many coins (exact amount depending on available memory), will exhaust the available memory usage. There are also likely performance impact that could be improved.

Solutions

Index coins by age and value for the walletdb, and load only the necessary coins into memory. Random selection can be done by choosing random min value with a range query, and limit of 1.

[braydonf]

As part of resolving this, it'll be useful to update the GET /wallet/:id/coin endpoint to support querying sets of results (pages), as is similar to GET /tx/address/:address.

With this issue solved, it could be worthwhile to only mark a coin/credit as spent (and not delete) when a block is confirmed, as is currently the case for unconfirmed transactions for a wallet.