address client wallet hanging issues

[chappjc]

We're clearly fighting some limitations of our asset.Wallet interface (no ctx args) and Go (no context.Merge), but there are some smaller things we can do.

The first commit adds some more context passing to avoid things getting hung and unable to cancel them. Some sensible timeouts are also established.

The second commit is a bit of a hack to prevent shutdown from becoming impossible if an RPC or something hangs and causes deadlock on the trackedTrade mutex.

This PR is the basis for more extensive refactoring in #1739

[chappjc]

Ultimately we probably should move toward adding ctx args on the interfaces like client/asset.Wallet and client/asset/btc.Wallet. Things would be a whole lot easier if there were a context.Merge, but that doesn't exists so I guess we should give up on trying to use an asset's own subsystem context and just accept the context from the caller i.e. Core.

Another challenge is how we do a TON with the trackedTrade mutex locked in (*Core).tick. Any wallet request hang is deadly because of that, not just because of logout, but many other handlers. That'll take a bit more careful refactoring, which we can do after branching release-v0.5. EDIT: A resolution to these design challenges is now in #1739