Fix validator build block when syncing

[DarianShawn]

Description

Validator nodes build blocks during sync, which makes it slow to sync and full of error logs.
This PR fixes it to only start building blocks when syncing is done.

Changes include

• Bugfix (non-breaking change that solves an issue)

Testing

• I have tested this code with the official test suite
• I have tested this code manually

Manual tests

1. start up a 4-validators network.
2. stop a validator for about 2-3 minutes.
3. restart the stopped validator and check its logs.

On current branch, validator will print out "sequence started" only when it catch up to the latest height.
On the target branch, validator will print out "sequence started" every time a new block arrives, and try to build an outdated block every turn.

[codecov-commenter]

Codecov Report

Merging #313 (6b408d2) into dev (a840ee8) will decrease coverage by 3.54%.
The diff coverage is 41.12%.

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@@            Coverage Diff             @@
##              dev     #313      +/-   ##
==========================================
- Coverage   51.21%   47.67%   -3.54%     
==========================================
  Files         117      131      +14     
  Lines       17525    19836    +2311     
==========================================
+ Hits         8975     9457     +482     
- Misses       7789     9561    +1772     
- Partials      761      818      +57     

Impacted Files	Coverage Δ
blockchain/storage/kvstorage/kvstorage.go	54.16% <ø> (+2.46%)	⬆️
chain/chain.go	28.69% <ø> (ø)
consensus/ibft/hooks.go	55.26% <ø> (ø)
consensus/ibft/pos.go	0.00% <0.00%> (ø)
helper/keccak/keccak.go	0.00% <ø> (ø)
helper/keccak/pool.go	0.00% <ø> (ø)
jsonrpc/debug_endpoint.go	36.08% <0.00%> (-0.76%)	⬇️
jsonrpc/net_endpoint.go	0.00% <0.00%> (ø)
network/bootnodes.go	85.71% <ø> (-14.29%)	⬇️
network/connections.go	98.07% <ø> (ø)
... and 131 more

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[0xcb9ff9]

LGMT