Move account_member_index to a plugin

[abitmore]

account_member_index is only being used in API nodes. Block producing nodes and seed nodes don't need it. We can save RAM and CPU if it's moved to a plugin (by not loading it).

[jmjatlanta]

Note: I am not claiming this issue. I am just gathering requirements.

Programming tasks:

1. Create a plugin that adds the secondary index account_member_index to the account_index.
   • the account_member_index should no longer be added in db_init.cpp.
2. Load this new plugin if the application has (or will have) an rpc-endpoint or rpc-tls-endpoint, as this indicates it is a node that provides API access.

Testing tasks:

1. Verify that a node that has no rpc endpoint does not have the secondary index
2. Verify that a node that has an rpc endpoint has the secondary index, and that a query that uses that index can find entries successfully.

---

Edit: updated based on 1 comment below.

[pmconrad]

1. An API node is a node that is listening on a rpc-endpoint or rpc-tls-endpoint
2. This should work without a replay. Upon shutdown, only the objects in the index are dumped. On the next startup the index (and its secondaries) are populated by reading the objects from disk.

[jmjatlanta]

A SWAG of an estimate:

• Buildout of plugin: 0.5h
• Removal of index in db_init.cpp and addition of index in plugin: 1h
• Load plugin at correct time 1h
• Create and complete tests 2h

Please provide your comments, and I will adjust.

[abitmore]

To got this done, we may need to adjust implementation of certain API's, e.g. check if the plugin is loaded and etc.

However, as mentioned in #683, this will result only 2-3% performance gain (measured in time). I don't know how much RAM it will save, guess it shouldn't be significant since a node enabled witness plugin only now uses 1.5G RAM only. Then we decide whether do it now, likely the answer is "no" or "low priority".

[pmconrad]

we may need to adjust implementation of certain API's, e.g. check if the plugin is loaded and etc.

It would be generally useful if libraries/application could automatically make use of APIs provided by the enabled plugins, without requiring knowledge (or specific code) for these APIs. See also #65 .

[pmconrad]

Resolved by #1918