refactor!: remove unused escrow handler methods

[gusinacio]

EscrowHandler was supposed to keep track of the escrow balance and keep it under the maximum amount willing to lose. Managers are created to be one per Allocation, whereas the escrow balance is shared between all allocations for a given sender.
This was replaced by the Tracker in tap-agent, and we are removing now that methods since it doesn't fit.

• Rename EscrowHandler to SignatureHandler
• Remove subtract_escrow and get_available_escrow
• Remove AwaitingReserve receipt state
• Rename Reserved to Checked receipt state

[coveralls]

Pull Request Test Coverage Report for Build 12932543202

Details

• 16 of 19 (84.21%) changed or added relevant lines in 3 files are covered.
• 22 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-2.0%) to 81.505%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tap_core/src/manager/adapters/signature.rs	8	11	72.73%

Files with Coverage Reduction	New Missed Lines	%
tap_core/src/manager/context/memory.rs	22	81.31%

Totals
Change from base Build 12931811666:	-2.0%
Covered Lines:	1040
Relevant Lines:	1276

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💛 - Coveralls