client/{core,db}: headless options

[chappjc]

These options are aimed at improving startup and shutdown speed of headless clients by avoiding unneeded work.

client/{core,db}: options for core startup/shutdown

This adds two new core.Config options aimed at improving startup
and shutdown depending on the user's use case:
 - NoAutoWalletLock instructs Core to skip locking the wallet on
   shutdown. This can be helpful if the user wants the wallet to remain
   unlocked. e.g. They started with the wallet unlocked, or they intend
   to start Core again and wish to avoid the time to unlock a locked
   the wallet on startup.
 - NoAutoDBBackup instructs the DB to skip the creation of a backup DB
   file on shutdown. This is useful if the consumer is using the
   BackupDB method, or simply creating manual backups of the DB file
   after shutdown.

These options are intended for direct Core consumers, such as headless
applications that drive Core directly instead of via the browser UI.

These options are created so that the zero-value corresponds to legacy
behavior. That is, the defaults are the existing behavior. However, in
client/db/bolt, the BackupOnShutdown is reversed from Core's
NoAutoDBBackup flag. The NewDB constructor accepts an optional db.Opts
struct, and if it is not provided, a defaultOpts is used that has
BackupOnShutdown set to true.

client/{asset,core}: add UnlockCoinsOnLogin option

This adds the core.Config.UnlockCoinsOnLogin option that indicates that
on wallet connect during login, or on creation of a new wallet, all
coins with the wallet should be unlocked.

To support this "unlock all" with the wallets, the ReturnCoins method
of the asset.Wallet interface now interprets a nil Coins slice to mean
that all coins should be unlocked. This is similar to the lockunspent
RPC's semantics.

The individual implementations of ReturnCoins are modified to recognize
a nil unspent input slice and accordingly request the wallet backend
unlock all coins.

[chappjc]

Working as intended. Added dexc command line switches, but the intended use is directly via Core (Go consumers)

[buck54321]

Can't seem to get --no-wallet-lock to work.

1. Client running with flag, connected to dcr rpc alpha node
2. Unlock wallet.
3. In harness, ./alpha walletislocked returns false
4. Shut down dexc
5. In harness, ./alpha walletislocked returns true

[buck54321]

Maybe

if err := btc.node.lockUnspent(true, nil); err != nil {
	return err
}
btc.fundingCoins = make(map[outPoint]*utxo)
return nil

[chappjc]

Good idea. going to apply that in the selective utxo loop below too.

[buck54321]

On wallet creation too?

[chappjc]

Correct. It's in the docs and help for the cli switch. You mean the field name should change? UnlockCoinsOnInitialAccess? The most routine use is login of course.

I didn't really want to wire this in to the connect helpers.

[chappjc]

Hmm, That's strange. I did verify that with the flag, the message c.log.Infof("Locking %s wallet", symb) was not printed out. Will check if there's locking going on elsewhere. Did you note the same change in the logs?

Oh dear, we've always been locking wallets twice it seems. Once on Run shutdown and once in Logout as well. :/

[chappjc]

I'm undecided about the last commit ab7660f. Has potential for gotchas, and it's code that hasn't been giving us trouble thus far. If the work in this PR finds it's way back to release-v0.5, I don't want it with mods to that part of the code.