Optimized address service mempool index size

[braydonf]

Closes #324

New features:

• significant memory footprint improvements
• address service now keeps mempool index with levelup that can be configured to use memdown or leveldown (default)
• the mempoolSpentIndex has been kept to be memory only and the key will be 37 bytes
• mempool address index size should be roughly half the size (binary encoding instead of utf-8)
• mempool indexes are more accurately kept in sync by using tx and txleave events from bitcoind and will nolonger make an expensive query for all transactions in the mempool

Includes breaking API changes:

• address service resetMempoolIndex method have been removed
• db service runAllMempoolIndexes has been removed

[kleetus]

LGTM

[pnagurny]

would be nice not to have these prefixes hardcoded

[braydonf]

Which prefixes?

[braydonf]

This also mirrors: https://github.com/bitpay/bitcore-node/blob/master/lib/services/db.js#L75

[pnagurny]

Right, instead of having to write this same code block multiple times, it could be DRY'ed up. Not a show stopper, we can clean it up later.

[pnagurny]

The prefixes I was talking about were {livenet: '', testnet: 'testnet3', regtest: 'regtest'}.

[braydonf]

Since there is some shared code, it could be useful to have this.node.networks.getBaseDataDir() or at least a utility function on node, we can spawn an issue and add it in later, since this should also touch the db.js code.

[pnagurny]

LGTM