Use BufWriter for state dump

[mikhailOK]

On my machine this speeds up binary dump from 13.53s to 1.17s, json dump from 117s to 4.5s

[bowenwang1996]

We can merge this to staging as it is not needed immediately.

[mikhailOK]

could be one of the reasons why 1 hour testnet state dump was observed

[codecov]

Codecov Report

Merging #2122 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2122      +/-   ##
==========================================
- Coverage   86.65%   86.62%   -0.04%     
==========================================
  Files         166      166              
  Lines       31738    31661      -77     
==========================================
- Hits        27503    27426      -77     
  Misses       4235     4235              

Impacted Files	Coverage Δ
chain/client/tests/cross_shard_tx.rs	17.12% <0.00%> (-1.25%)	⬇️
chain/client/tests/catching_up.rs	10.58% <0.00%> (-1.04%)	⬇️
...enesis-tools/genesis-csv-to-json/src/csv_parser.rs	68.34% <0.00%> (-0.43%)	⬇️
chain/client/tests/query_client.rs	93.33% <0.00%> (-0.42%)	⬇️
near/tests/sync_state_nodes.rs	55.14% <0.00%> (-0.42%)	⬇️
runtime/runtime/src/balance_checker.rs	96.70% <0.00%> (-0.37%)	⬇️
chain/client/src/client_actor.rs	84.72% <0.00%> (-0.28%)	⬇️
chain/chain/src/test_utils.rs	79.70% <0.00%> (-0.19%)	⬇️
chain/client/tests/process_blocks.rs	79.33% <0.00%> (-0.17%)	⬇️
chain/client/tests/chunks_management.rs	96.66% <0.00%> (-0.13%)	⬇️
... and 25 more

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