Provide a way to transfer whole balance on-chain

[andrevmatos]

Description

As Kelsos, I want to be able to empty an account (e.g. subkey) of ETH and tokens, by transfering entire balance to some other account (e.g. main account).
There're a couple of long-standing TODO in source for this.

light-client/raiden-ts/src/raiden.ts

Line 1096 in 39dd196

* TODO: expose a nice way to transfer ALL, considering gas price & limit

light-client/raiden-ts/src/raiden.ts

Line 1125 in 39dd196

* TODO: expose a nice way to transfer ALL tokens

Needed for #1365

Acceptance criteria

• I can call SDK's method with special parameters to specify I want to transfer entire balance to another account
• I can call SDK's method with special parameters to specify I want to transfer entire ETH balance to another account
  • This is trickier, as SDK needs to consider gas cost on this last tx, so the tx succeeds and the remaining balance is zero

Tasks

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