Replaced sync method modifyBody with async

Replaced syncronous method modifyBody()
and syncronous methods convertToMediawiki(),
convertToMWDeprecated() and convertToZotero()
with asyncronous methods selectFormatFcn(),
convertToMediawikiAsync(), convertToMWDeprecatedAsync(),
and convertToZoteroAsync.

Converted associated citation property methods
to async pattern as well, such as fixAccessDate(),
fixURL, addPMID(), etc.

Fixes race condition bug apparent in the /api endpoint.

Change-Id: Icaef5a5f74fb240e9c45892b0c9d2f1c779dff38