Allow pika to automatically create mpi pool on startup

[biddisco]

Add a pool create option to 'init params' - this allows pika to call mpi pool_create at startup and remove the need for the user to setup a polling pool

Move many functions into detail:: namespace to designate them as being for internal use only and not for general user consumption

Clean up some of the enum flags and options that are used to setup pools and polling modes etc.