EOSIO PRs from MEET.ONE

Author: UMU @ MEET.ONE Lab 支持我们，请投票给 rex.m

1. eos

By @UMU618

change core symbol name length to 7(MEETONE) #5958

fix issue 5488 #6498

mongodb shard: line 870, code 61, generic server error #5488

Optimize mongodb plugin #6990

Replace hard coding system_account_name #8254

Remove Unused Variable #8269

2. eosio.cdt

By @UMU618

support std::set<eosio::permission_level> #200

3. eosjs

By @wuyahuang

add get_table_by_scope to eosjs #514

By @UMU618

replace get_block with get_block_header_state #582

Suggest replacing get_block() #581
transact() can fail because of #582 #584

Remove unnecessary call to get_block_header_state #703

4. eosjs-api

By @UMU618

Add API: getTableByScope #19