allow splitting withdrawal txs

[jleni]

Due to HSM limitations (Ledger Nano S), big transactions such as withdrawing rewards from a large number of validators, may be rejected to due memory constraints.

This PR introduces an additional flag that allows splitting the withdrawal messages and generates several transactions. Users can then define the maximum number of messages that they want to use.

This is related to: https://github.com/cosmos/ledger-cosmos/issues/143
While we have extended limits to Nano X in the next release, it is important to provide an alternative to users of Nano S.

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[alexanderbez]

One minor nit, otherwise LGTM 👌

I may actually renege my statement on making this functionality only available to this command. Would you still be in favor of applying this to all commands?

[rigelrozanski]

good changes! should tidy the code just a wee bit