Some floats cast as strings in LVAlerts and response JSONs

As reported in emfollow/gwcelery!94 (merged), there are situations in which certain float variables are being sent as strings in LVAlert JSONs and even in some response JSONs from gracedb-client.

I believe this is happening because in some of the old code in the events API, the event is converted to a dictionary BEFORE it is saved in the database. Thus, some variables which are extracted from the event file (as strings) are still in memory as strings. If we did the save first, that would cast those variables to the correct type.

I will submit some test events for the active pipelines (gstlal, gstlal-spiir, pycbc, mbtaonline, cwb, lib, swift, fermi, and snews) and check the LVAlert content, as well as the response in gracedb-client.