The secret spend key is kept encrypted in memory, and
decrypted on the fly when needed.
Both spend and view secret keys are kept encrypted in a JSON
field in the keys file. This avoids leaving the keys in
memory due to being manipulated by the JSON I/O API.
Fixes failing test during Arch package build (due to attempt to write to
~/.bitmonero/...).
Prefix temp dir path with "monero-" because we are not putting it on the
system, so good to identify ourselves in case the dir gets left over due
to crash, etc.
