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.
Inherited db::replayer_service from db::high_priority_service
and overrode after_apply() and is_replaying() methods to match
current state of library implementation. Changed
db::client_service::replay() to use db::replayer_service instead
of db::high_priority_service().
Depending on the DBMS client session allocation strategy the
client id may or may not be available when the client_session
is constructed, therefore there should be a method to assign
an id after construction. Close/cleanup methods were added to
clean up open transactions appropriately.
