Do we have a SSR relay technology that can stand 100 switching per second for a long time?
I want to use relays for 5vdc control and 12vdc output with very low
  current. But I don't know if it is safe to use it or not.


If you are switching DC then you don't need an SSR - use a BJT or a MOSFET. Consider a MOSFET that is used in an off-line switching AC to DC converter - it might be switching 100,000 times per second and for 10+ years - is this enough?

Consider also that is it switching dc currents of a few amps at voltages up to maybe 400 V.

Also what about a class C RF amplifier operating at maybe 300 MHz - its output transistor is switching power at 300,000,000 times per second for possibly several years also.

If you do need galvanic isolation then a photo-voltaic optical coupler can be used to activate/deactivate the MOSFET exactly the same way SSRs use them. Same technology same long-life and gazillions of operations.