Log informational message when plugin starts

[jmjatlanta]

Fixes #1459

Once a plugin starts, an informational message is added to the log, showing the name of the plugin and that it has started. Example:

674910ms th_a       application.cpp:196           reset_p2p_node       ] Adding seed node 144.202.100.49:1776
674911ms th_a       application.cpp:211           reset_p2p_node       ] Configured p2p node to listen on 0.0.0.0:43911
674912ms th_a       application.cpp:1156          startup_plugins      ] Plugin account_history started
674912ms th_a       application.cpp:1156          startup_plugins      ] Plugin grouped_orders started
674912ms th_a       application.cpp:1156          startup_plugins      ] Plugin market_history started
674912ms th_a       witness.cpp:118               plugin_startup       ] witness plugin:  plugin_startup() begin
674912ms th_a       witness.cpp:139               plugin_startup       ] No witness configured.
674912ms th_a       witness.cpp:141               plugin_startup       ] witness plugin:  plugin_startup() end
674912ms th_a       application.cpp:1156          startup_plugins      ] Plugin witness started

[oxarbitrage]

I like that, much easier than adding to each plugin as i suggested.

by the way, you can maybe take a look at #1437