tx fees, policy: CBlockPolicyEstimator update from CValidationInterface notifications

`CBlockPolicyEstimator` will implement `CValidationInterface` and
subscribe to its notification to process transactions added and removed
from the mempool.

Re-delegate calculation of `validForFeeEstimation` from validation to fee estimator.

Also clean up the validForFeeEstimation arg thats no longer needed in `CTxMempool`.

Co-authored-by: Matt Corallo <git@bluematt.me>

src/kernel/mempool_options.h

@@ -13,8 +13,6 @@
 #include <cstdint>
 #include <optional>
 
-class CBlockPolicyEstimator;
-
 /** Default for -maxmempool, maximum megabytes of mempool memory usage */
 static constexpr unsigned int DEFAULT_MAX_MEMPOOL_SIZE_MB{300};
 /** Default for -maxmempool when blocksonly is set */
@@ -37,8 +35,6 @@ namespace kernel {
  * Most of the time, this struct should be referenced as CTxMemPool::Options.
  */
 struct MemPoolOptions {
-    /* Used to estimate appropriate transaction fees. */
-    CBlockPolicyEstimator* estimator{nullptr};
     /* The ratio used to determine how often sanity checks will run.  */
     int check_ratio{0};
     int64_t max_size_bytes{DEFAULT_MAX_MEMPOOL_SIZE_MB * 1'000'000};