u_flushCache() to close files, release memory

Provide one or more functions like u_flushCache() that

• release memory

• close file handles

• be thread-safe

• only releases objects that are not in current use

See the current converter-service-only ucnv_flushCache().

Releasing file handles allows to replace data files. Releasing memory reduces
the current runtime footprint.

We could add one such functions for all services and/or one function per
service.