eth, cli: prevent snap sync mode migration - v0.2.x

[manav2401]

We've seen a couple of scenarios where some of our internal nodes have migrated to snap sync mode. Until we make it working fully, this PR will prevent that from happening.

It adds a preventive layer in the cli (old one) so that it's not enabled through flag and also handles cases where is migrates to snap sync.

[codecov-commenter]

Codecov Report

Merging #495 (d6c3972) into v0.2.17-candidate (49cd8c8) will increase coverage by 0.03%.
The diff coverage is 0.00%.

@@                  Coverage Diff                  @@
##           v0.2.17-candidate     #495      +/-   ##
=====================================================
+ Coverage              56.76%   56.79%   +0.03%     
=====================================================
  Files                    578      578              
  Lines                  68317    68307      -10     
=====================================================
+ Hits                   38778    38795      +17     
+ Misses                 26182    26166      -16     
+ Partials                3357     3346      -11     

Impacted Files	Coverage Δ
eth/handler.go	54.15% <0.00%> (+0.19%)	⬆️
eth/sync.go	58.69% <ø> (+5.23%)	⬆️
p2p/discover/v4_udp.go	72.75% <0.00%> (-4.35%)	⬇️
eth/protocols/snap/sync.go	70.37% <0.00%> (-1.36%)	⬇️
les/downloader/downloader.go	75.15% <0.00%> (-0.11%)	⬇️
trie/proof.go	75.00% <0.00%> (ø)
p2p/discover/v5_udp.go	81.21% <0.00%> (ø)
core/tx_pool.go	71.48% <0.00%> (+0.27%)	⬆️
les/test_helper.go	88.82% <0.00%> (+0.28%)	⬆️
eth/downloader/downloader.go	72.28% <0.00%> (+0.35%)	⬆️
... and 15 more

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