Introduce explicit -walletupgrade option

Do not automatically change the wallet format unless the user takes an
explicit action that implies an upgrade (encrypting, for now), or uses
-walletupgrade.

-walletupgrade optionally takes an integer argument: the client version
up to which upgrading is allowed. Without an argument, it is upgraded
to latest supported version. If an argument to -walletupgrade is
provided at the time the wallet is created, the new wallet will initially
not use features beyond that version.

Third, the current wallet version number is reported in getinfo.

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