Add UUID support to DuckDBAppender (fixes #313)

[arouel]

Previously, attempting to append a UUID would result in an unsupported C API type: 27 error which is documented in #313. This change enables the DuckDBAppender to handle java.util.UUID objects directly.

[staticlibs]

Hi, thanks for the PR, it looks good to me! To fix the formatting:

python -m pip install --user clang_format==11.0.1
make format
make format-check

[arouel]

@staticlibs thank you for the quick feedback. I refined the formatting.

One quick question: I stumbled upon the appenderRefLock in the new DuckDBAppender and asked myself for what we need a Lock, because a DuckDBAppender cannot be used by multiple threads anyways?

[staticlibs]

@arouel

for what we need a Lock, because a DuckDBAppender cannot be used by multiple threads anyways

The Appender is not intended to append the data concurrently, but it can be closed concurrently. For example, if the Appenders's connection is closed, the Appender is closed automatically. This connection close can well be triggered concurrently, for example from the shutdown cleanup code. After the Appender is closed - its native part is destroyed. And the destruction of the native part cannot be concurrent with it usage from other thread. Almost all append() methods (with the notable exception of strings over 12 bytes long) are Java-local - they only lay the input data out into the Data Chunk (thus they should be fast). But flush() operation calls to the native part and cannot be concurrent with close(). So the appenderRefLock is used to put them in order.

[arouel]

Thank you for the thorough response.