Unify major syncing detection

[keorn]

Improves #2639.
Tested on two nodes in various sync conditions (early, mid, import), it is reliable when repeatedly querying RPC. Still returns false for a few seconds of initial sync, but thats rather unavoidable.
Should also improve situations similar to the attack when lots of uncles were increasing queue size, possibly leading to "Syncing" console output.

Its not used for mission critical stuff so block delay can be adjusted later (4 behind now).

[coveralls]

Changes Unknown when pulling 71281cc on eth-syncing into * on master*.

[tomusdrw]

RPC test is failing. Looks good apart from that.

[coveralls]

Coverage decreased (-0.03%) to 86.225% when pulling 3ad8c4f on eth-syncing into b039323 on master.

[arkpar]

This does not seem right to me. Block queue can still have a few thousand blocks to process after the sync modules completes a download. eth_syncing should return false only after the block queue has been cleared out.

[keorn]

In this case isn't it SyncState::Waiting? SyncState::Waiting implies major sync here.

[arkpar]

No, the sync does not track the queue status after it is done downloading blocks. That's why there was a check for the queue status in the original code.