Make the `VALUE` union definition private and consistent

[ralphlange]

I notice another bit of weirdness. The VALUE union definition is also conditional on DBR_INT64, but does not directly include any header which defines it. To make qtcreator shut up about this I tried adding #include <dbAccess.h> to caPutLogTask.h. This led me to realize that caPutJsonLogTest.cpp includes db_access.h, which does not defined DBR_INT64. So different files within caPutLog have different views of this union. Not practically a problem since the double member has the same size and (I think) the same or stricter alignment. Still, exposing this union seems a magnet for ABI problems.

Originally posted by @mdavidsaver in #48 (comment)