fixed a typo.
protocol decoder. refactored unit test.
mail store. refactored object initialization.
message db. defined message exist test API.
refactored required libraries at db unit test.
meta db. refactored some code format.
meta db. modified mailbox collection API. optional mailbox id parameter at mailbox add method.
meta db. modified message date attribute.
no error to clear nothing date entry.
meta db. defined dirty flag.
core db. defined test read API.
meta db. defined API set of recovery.
meta db. implemented recovery phase 1.
meta db. implemented recovery phase 2.
meta db. implemented recovery phase 3.
meta db. implemented recovery phase 4.
meta db. implemented recovery phase 5.
meta db. implemented recovery phase 6.
meta db. implemented recovery phase 7.
meta db. implemented recovery phase 8.
meta db. added test case of recovery scenario.