chore(server): Eliminate race conditions in version info fetch.
chore(server): Eliminate race conditions in version info fetch.

Use promises to ensure no race conditions exist when fetching
version info. Version info will only be fetched once and then re-used
for every request.

TDA/fxa-two-factor-auth,shane-tomlinson/fxa-content-server,shane-tomlinson/fxa-content-server,vladikoff/fxa-content-server,atiqueahmedziad/fxa-content-server,ofer43211/fxa-content-server,mozilla/fxa-content-server,TDA/fxa-content-server,ReachingOut/fxa-content-server,vladikoff/fxa-content-server,TDA/fxa-content-server,vladikoff/fxa-content-server,swatilk/fxa-content-server,npestana/fxa-content-server,TDA/fxa-content-server,ReachingOut/fxa-content-server,swatilk/fxa-content-server,mozilla/fxa-content-server,mozilla/fxa-content-server,npestana/fxa-content-server,atiqueahmedziad/fxa-content-server,npestana/fxa-content-server,shane-tomlinson/fxa-content-server,atiqueahmedziad/fxa-content-server,ofer43211/fxa-content-server,ReachingOut/fxa-content-server,TDA/fxa-two-factor-auth,ofer43211/fxa-content-server,TDA/fxa-two-factor-auth,swatilk/fxa-content-server