No real changes, just make it possible to use these helpers in the unit
test to be added.

This commit is best viewed using Git --color-moved option.

This will allow to use it for creating functions as well as procedures
(and also procedures with a name other than "soci_test" if this is ever
needed).

We need to read the entire contents of the CLOB in Oracle backend and
not just the number of bytes corresponding to its length in characters
as returned by OCILobGetLength() because this may (and will) be strictly
less than its full size in bytes for any encoding using multiple bytes
per character, such as the de facto standard UTF-8.

Also make reading CLOBs more efficient by doing what Oracle
documentation suggests and using the LOB chunk size for reading.

Finally, add a unit test checking that using non-ASCII strings in UTF-8
(which had to be enabled for the CI) with CLOBs does work.

This commit is best viewed ignoring whitespace-only changes.

On "streaming mode" the offset is ignored, except for the first call, so
don't bother updating it.

Also restructure the code in a slightly simpler way.

Read directly into the provided string instead of reading into a
temporary buffer and then copying into the string.