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.
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.
