replace tx-sender with contract-caller where appropriate

[pseudozach]

TMS-2
File(s):
Trustless-rewards.clar, triggerswap_v6.clar, stxswap_v10.clar, sip10swap_v3.clar
Recommendation: Use variable contract-caller that is local and it cannot be improperly encapsulated as tx-sender can be. In some cases with transfer the solution to this problem is to use the function as-contract to limit tx-sender and contract-caller to the value of the contract context.