Public headers may be compiled with different preprocessor
options than compilation units or private headers within a
project. Introduced a macro WSREP_ASSERT() to be mainly used in
public headers. The macro can be enabled by defining
WSREP_LIB_HAVE_DEBUG and is enabled by default for Debug type builds.
There are some corner cases where keys with two parts are needed
for a transaction. Relaxed the assertion and sanity check so that
at least two key parts are needed for each key which is assigned
to a transaction.