Use soft values in ValueTypeCodec

Use soft values in ValueTypeCodec

Using strong value references in STATIC_CODECS cache leads to target
classes being strongly reachable through the codec itself -- hence
they will never be weakly reachable, hence they will not be GC'd.

Use soft values, which allows the classes to be GC'd as soon as the
JVM decides they have not been used for a while -- which is what
will happen when a class becomes almost-eligible for unloading.

JIRA: MDSAL-580
Change-Id: I7956d564c46a9a1d52ba85fbe61f3a19c507a902
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>