Using printf will try to parse the string and replace special
characters. In migrate code, we want to just output the raw
json string of client.Node.
For example,
Printf("%\\") => %!\(MISSING)
Print("%\\") => %\
Thus, we should use print instead.
Migrate command accepts a datadir and an optional user-provided
transformer function that transform v2 keys to v2 keys.
Migrate command then builds a v3 backend state based on the existing
v2 keys and the output of the transformer function.
