Remove obsolete methods

[NicolasDorier]

• Remove the CoverOnly/CoverTheRest/ContinueBuild of TransactionBuilder.
• Make IDestination implementation explicit in Key and PubKey to discourage use of Key/PubKey.ScriptPubKey.
• Remove some old overload only present for backward compatibility
• Add PubKey.TryCreatePubKey and remove Pubkey.Check (deep check). Now we have PubKey.SanityCheck for quick sanity check.
• Remove GetSegwitAddress
• Add some nullable context, as refactoring of PubKey and Key made it easy to support it.
• Use IAddressableDestination rather than TxDestination when possible
• Remove TxDestination
• Remove BitcoinAddress.GetScriptAddress
• Remove ScanTxoutSetRequest
• Remove obsolete Schnorr signing