feature: validate term value

[GuyLewin]

Fixes #511

[GuyLewin]

@BusyJay do you agree with my change in src/storage.rs?:

raft-rs/src/storage.rs

Lines 422 to 423 in b0a3740

	// Initialize term with a valid value (anything except 0)
	core.raft_state.hard_state.term = 1;

?

If so - it breaks a lot of tests but I can go over them and fix them. I just don't want to do it if it's the incorrect fix since it'll require some manual work.

[BusyJay]

MemStorage is supposed to be used for tests, change it or not are OK to me.