Turn MaxScale into a library.

Plugins loaded by MaxScale obviously need symbols provided by MaxScale. Currently plugins are not linked with "-Wl,-z,defs" (that would require all symbols to be resolved at link-time), which means that any missing symbols are detected only at runtime. In practice this may mean that a particular problem with an undefined symbol is detected only when the MaxScale test-suite is run for a particular OS. That is, it may take hours before that is detected. To be able to use "-Wl,-z,defs" MaxScale must be separated into a MaxScale library and a small MaxScale executable that is linked to that library. All plugins then link to that library as well.