Check allowance before depositing and skip token.approve

[andrevmatos]

Description

Very small issue. We should check token's allowance before depositing (to UDC deposit, on channelOpen or channelDeposit. If it's enough, we should skip approve and deposit directly.
Metamask's UI allows the user to edit the allowance required (instead of the standard value provided to approve calls, which are usually exactly the amount to deposit in the LC). If we do check it, users planning to do multiple deposits (e.g. for tests) can edit the allowance on Metamask and put a (possibly very) high value there, and next deposits would only require the actual beneficiary contract transaction (UDC or TokenNetworks), saving a possibly long waiting time for approval.
Note that, if editing in Metamask UI, it's advisable to also increase in a couple of thousand the gas limit (advanced gas edit), as a different value can cause a different gas requirement and could make tx to fail.

Acceptance criteria

• Deposit operations in SDK checks for allowance and skip approval if enough

Tasks

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