Use shared_ptr to WS connection in API connection

[abitmore]

For bitshares/bitshares-core#1690

My opinion: don't merge this unless we've found a way to fix the side effects: bitshares/bitshares-core#1695 (comment)
Update: already fixed the side effects along with some other CLI improvements, E.G.

• pressing TAB key will complete a command as long as possible
• Increased CLI command history buffer size to 256 (from 15)
• CLI no longer stores continuous duplicate commands to history, so no longer need to press UP key several times to find a different command
• pressing Ctrl+C will quit CLI cleanly at most time
• sending SIGINT or SIGTERM to CLI will terminate it cleanly at most time
• when CLI got disconnected by the API node, it will quit cleanly at most time

By the way,

• do you know we can press Ctrl+R to search in command history? Although it's a bit buggy.
• Do we want a feature that saves command history to a file e.g. on quit or on the fly and load it on next startup?

[abitmore]

When using a reference (the old code), it's possible that the referenced websocket_connection object has been destructed elsewhere when the websocket_api_connection object trying to dereference it, e.g. when calling websocket_api_connection::send_call(), which would cause a memory violation error.

After changed to a shared_ptr (this PR), it's guaranteed that the websocket_connection object will be always alive.

[abitmore]

Fixed signal handling in CLI. Need troglobit/editline#29.

[abitmore]

I think this PR is ready for merge, although there are still a few issues existing in editline (e.g. troglobit/editline#26, troglobit/editline#30, troglobit/editline#31), we can bump editline later when they're fixed, but there is nothing we can do right now.

By the way, check_secret is removed due to dumping of editline. If there is no progress on #97, I'll work on it.

OP updated.