IoTeX Dev Update Week 02/17/2020

[Yutong]

• native go staking work in process (#1893 #1900 #1901 #1902)
• add block migrator (#1879)
• introduce system log (#1891)
• code refactoring (#1858 #1881 #1894 #1896 )
• bug fixes (#1904 #1905 #1907 #1906)