Implement xpGetUserMeta procedure

[KallynGowdy]

Name: xpGetUserMeta
HTTP: GET /api/v2/xp/user/meta
Authentication: REQUIRED - sessionKey
Input: none
Operation: Get XpUser metadata by sessionKey.userId. Should validate the user ID and return information about the user's XpUser. The returned information should be limited to the information specified below.
Success Output:

{
    success: true,
    accountBalance: 123, // 0 if not a coach
    accountCurrency: 'usd',
    requestedRate: 0,
    ...getUserInfo(userId)
}