This fixes a deadlock where failed transactions due to the namespaces'
bucket being missing would cause deadlocks due to bolt's mmap rwmutex
still being read or write locked (and no way to unlock it, since the
underlying bolt tx was not returned on failure).
This commit implements a new namespaced db package which is intended to
be used be wallet and any sub-packages as its data storage mechanism.
- Key/value store
- Namespace support
- Allows multiple packages to have their own area in the database without
worrying about conflicts
- Read-only and read-write transactions with both manual and managed modes
- Nested buckets
- Supports registration of backend databases
- Comprehensive test coverage
