Persist API-configured voting settings:

Prior to this commit, the amendments that a server would vote in support
of or against could be configured both via the configuration file and
via the command line "feature" command. Changes made in the configuration
file would only be loaded once at server startup and changes made via the
command line take effect immediately but are not persisted across
restarts.

This commit deprecates management of amendments via the configuration
file and stores the relevant information in the `wallet.db` database
file.

1. On startup, the new code parses the configuration file.
2. If the `[veto_amendments]` or `[amendments]` sections are present,
   we check if the `FeatureVotes` table is present in `wallet.db`.
3. If it is not, we create the `FeatureVotes` table and transfer the
   settings from the config file.
4. Proceed normally but only reference the `FeatureVotes` table instead
   of the config file.
5. Warns if the voting table already exists in `wallet.db` and there
   exists voting sections in the config file. The config file is ignored
   in this case.

This change addresses & closes #3366

src/ripple/app/main/Application.cpp

@@ -1360,6 +1360,7 @@ ApplicationImp::setup()
         Section enabledAmendments = config_->section(SECTION_AMENDMENTS);
 
         m_amendmentTable = make_AmendmentTable(
+            *this,
             config().AMENDMENT_MAJORITY_TIME,
             supportedAmendments,
             enabledAmendments,