executor: introduce threads without coverage

Experiments have shown that the amount of allocated memory has a very
big impact on the syz-executor's performance (at least under Linux) -
much bigger than was expected.

One source of that extra virtual memory is kcov and, in fact, usually we
don't need all 16 kcov handles we create. E.g. only 4 are enough for 99.5%
progs that syzkaller executes. The biggest consumer of threads - the
collide mode doesn't need kcov at all.

Let kcov handle be an optional property of a thread, not a mandatory
one. Allocate only 3 kcov instances initially (they'll be preserved over
forks) and let the forked processes create other kcov instances if they
happen to be needed.

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