Add RPC that allows to change scheduler at runtime.

Change-Id: I008670f5e936bc25a0fbc923b826277d15343273
Signed-off-by: Vitaliy Mysak <[email protected]>
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/3958
Reviewed-by: Tomasz Zawadzki <[email protected]>
Reviewed-by: Jim Harris <[email protected]>
Tested-by: SPDK CI Jenkins <[email protected]>