Split bidi connection-stats code into a new C file.

src/core/mainloop/connection.c

src/core/mainloop/mainloop.c

src/feature/relay/relay_config.c

src/feature/stats/connstats.c

src/feature/stats/connstats.h

src/feature/stats/include.am

src/feature/stats/rephist.c

src/feature/stats/rephist.h

src/test/test_stats.c