Add estimateloan and estimatecollateral RPCs

[Jouzo]

What would you like to be added:

Add estimateloan RPC to calculate the amount of loan token a vault can take depending on collateral and loan scheme.
estimateloan loantoken vault
loantoken typed as string amount@token
vault should either be vaultId or (collateral amount, loan scheme) tuple

Add estimatecollateral RPC to estimate the collateral ratio.
estimatecollateral collateralamount scheme loantoken
or
estimatecollateral vaultId loantoken

Why is this needed:

As of now, a vault owner as no information as to how much loan token he can take.

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[prasannavl]

Fixed by #924, #927 and #933.