Convert server methods, add unit tests, and cleanup

src/api/common/index.js

@@ -2,11 +2,10 @@
 const utils = require('./utils');
 
 module.exports = {
-  core: require('./core'),
+  core: utils.core,
   constants: require('./constants'),
   errors: require('./errors'),
   validate: require('./validate'),
-  server: require('./server'),
   dropsToXrp: utils.dropsToXrp,
   xrpToDrops: utils.xrpToDrops,
   toRippledAmount: utils.toRippledAmount,