Use a lock-directory- instead of a lock-file- and backup prev wallet.dat. (#982)
Use a lock-directory- instead of a lock-file- and backup prev wallet.dat. (#982)

It looks like lockfile implementation did still suffer from a race
condition. Only when creating the file with O_EXCL, file creation fails
if the file does already exist. One could use O_EXCL, but Windows
might use another flag to achieve the same. Moreover, O_EXCL doesn't
work as expected if the files are accessed over NFS.
In contrast, mkdir() is atomic in every of the mentioned cases.
Aside from using a lockdirectory, this patch also backups the current
wallet.dat contents to wallet.bck in case writing a possibly hefty
amount of JSON to the new wallet.dat fails. It is hoped that at least
one of the .bck and .dat files has usable contents in case failure.
yeastplume/grin,yeastplume/grin