Stop finding paths when enough are found (RIPD-156)
Pathfinder cleanups, more efficient 'd' handling (RIPD-156)
Remove some dead SHAMapMissingNode code
Check for payment increments that make no progress. (RIPD-374)
Add more documentation of ledger acquisition (RIPD-373)

Capturing information from a seminar on the topic into the source tree.
Simplify locking and move a typedef.

* Make DatabaseCon's lock private and expose a scoped lock_guard.
* Get rid of DeprecatedRecursiveMutex and DeprecatedScopedLock entirely.
* Move CancelCallback to Job where it logically belongs.