syncthing: add GC and thread control variables

syncthing: add GC and thread control variables

Add Go GC and threading control variables - GOGC, GOMEMLIMIT, and
GOMAXPROCS - to all services to allow more granular control of the
memory management on lower memory devices.

Link: https://go.dev/doc/gc-guide#GOGC
Link: https://pkg.go.dev/runtime#pkg-overview
Link: https://go.dev/blog/container-aware-gomaxprocs
Signed-off-by: George Sapkin <redacted>