feat(service|btc)!: adapt org mempool updates in btc-assets-api

[ShookLyngs]

Changes

• Add BtcAssetsApi.getBtcRecommendedFeeRates() as the btc-assets-api service provides the API directly
• Add after_txid filter in BtcAssetsApi. getBtcTransactions() API to allow query starts after a btc txid
• Remove DataSource.getRecommendedFeeRates() and DataSource.getAverageFeeRate() API
• Set fastestFee as the default fee rate in TxBuilder (it was halfHourFee)
• Remove mempool.js from the service lib's dependencies
• Replace deprecated test info in the service tests
• Rename service tests to improve readability

Parent PR

Most of the updates in this pull request are based on the org mempool api updates in btc-assets-api:

• https://github.com/ckb-cell/btc-assets-api/pull/96

Related issues

• Resolves #143
• Resolves #92

TODOs

• [x] Add tests of the after_txid filter
• [x] Test the mempool api 503 error (see if still occurs)
• [x] Re-enable btc/service tests once everything's normal again #143