Revert "core/state/snapshot: simplify snapshot rebuild (#30772)"

[rjl493456442]

This reverts commit 2380012.

The original pull request introduces a bug and some flaky tests are detected because of this flaw.

--- FAIL: TestRecoverSnapshotFromWipingCrash (0.27s)
    blockchain_snapshot_test.go:158: The disk layer is not integrated snapshot is not constructed
{"pc":0,"op":88,"gas":"0x7148","gasCost":"0x2","memSize":0,"stack":[],"depth":1,"refund":0,"opName":"PC"}
{"pc":1,"op":255,"gas":"0x7146","gasCost":"0x1db0","memSize":0,"stack":["0x0"],"depth":1,"refund":0,"opName":"SELFDESTRUCT"}
{"output":"","gasUsed":"0x0"}
{"output":"","gasUsed":"0x1db2"}
{"pc":0,"op":116,"gas":"0x13498","gasCost":"0x3","memSize":0,"stack":[],"depth":1,"refund":0,"opName":"PUSH21"}

Before the original PR, the snapshot would block the function until the disk layer
was fully generated under the following conditions:

(a) explicitly required by users with AsyncBuild = false.
(b) the snapshot was being fully rebuilt or the disk layer generation had resumed.

Unfortunately, with the changes introduced in that PR, the snapshot no longer waits
for disk layer generation to complete if the generation is resumed. It brings lots of
uncertainty and breaks this tiny debug feature.

[MariusVanDerWijden]

LGTM

[MariusVanDerWijden]

cc @ARR4N we had to revert your commit, since it changed the logic a bit

[ARR4N]

Sorry!

[holiman]

LGTM