Make enter/leave calls optional for API3

This makes the usage of `lcb_sched_enter()` and `lcb_sched_leave()` optional. These APIs are there mainly to support the 'all-or-nothing' scheduling semantics of the V2 API and the Python SDK, both of which support so-called "multi" operations.

Upon the advice of Brett Lawson, the behavior has now been extended so that when an operation is added to the pending/spooling queue, it will be transferred to the request queue if a previous call to `lcb_sched_enter()` is not active. If a current call to `lcb_sched_enter()` is active then the request is only placed in the queue when `lcb_sched_leave()` is called.

This allows simple idioms like

lcb_CMDGET gcmd = { 0 };

LCB_CMD_SET_KEY(&gcmd, "key", 3);

lcb_get3(instance, NULL, &gcmd);

lcb_wait(instance);