Removed calls to assert() from public headers to have
full control when assertions are enabled in wsrep-lib
code regardless of parent project build configuration.
Moved methods containing assertions and non-trivial
code from headers into compilation units.
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.
