core: speed up header import (#21967)

This PR implements the following modifications

- Don't shortcut check if block is present, thus avoid disk lookup
- Don't check hash ancestry in early-check (it's still done in parallel checker)
- Don't check time.Now for every single header

Charts and background info can be found here: https://github.com/holiman/headerimport/blob/main/README.md
With these changes, writing 1M headers goes down to from 80s to 62s.

core/rawdb/freezer_table.go

@@ -103,6 +103,11 @@ type freezerTable struct {
 	lock   sync.RWMutex // Mutex protecting the data file descriptors
 }
 
+// NewFreezerTable opens the given path as a freezer table.
+func NewFreezerTable(path, name string, disableSnappy bool) (*freezerTable, error) {
+	return newTable(path, name, metrics.NilMeter{}, metrics.NilMeter{}, metrics.NilGauge{}, disableSnappy)
+}
+
 // newTable opens a freezer table with default settings - 2G files
 func newTable(path string, name string, readMeter metrics.Meter, writeMeter metrics.Meter, sizeGauge metrics.Gauge, disableSnappy bool) (*freezerTable, error) {
 	return newCustomTable(path, name, readMeter, writeMeter, sizeGauge, 2*1000*1000*1000, disableSnappy)