Add creation date in asset_object and account_object

User Story

As an API user I would like to know when an account or an asset was created.

Note: see #1198, must get the actual timestamp of the block.

Impacts Describe which portion(s) of BitShares Core may be impacted by your request. Please tick at least one box.

[x] API (the application programming interface)
[ ] Build (the build process or something prior to compiled code)
[ ] CLI (the command line wallet)
[ ] Deployment (the deployment process after building such as Docker, Travis, etc.)
[ ] DEX (the Decentralized EXchange, market engine, etc.)
[ ] P2P (the peer-to-peer network for transaction/block propagation)
[ ] Performance (system or user efficiency, etc.)
[ ] Protocol (the blockchain logic, consensus, validation, etc.)
[ ] Security (the security of system or user data, etc.)
[ ] UX (the User Experience)
[ ] Other (please add below)

CORE TEAM TASK LIST