dsync: Add per-mailbox sync lock that is always used. Both importing and exporting gets the lock before they even sync the mailbox. The lock is kept until the import/export finishes. This guarantees that no matter how dsync is run, two dsyncs can't be working on the same mailbox at the same time. This lock is in addition to the optional per-user lock enabled by the -l parameter. If the -l parameter is used, the same lock timeout is used for the per-mailbox lock. Otherwise 30s timeout is used. This should help to avoid email duplication when replication is enabled for public namespaces, and maybe in some other rare situations as well.

#ifndef STATS_CARBON
#define STATS_CARBON 1

struct stats_send_ctx;

int
stats_carbon_send(const char *endpoint, const char *data,
		  void (*callback)(void *), void *cb_ctx,
		  struct stats_send_ctx **ctx_r);
void
stats_carbon_destroy(struct stats_send_ctx **ctx);

#endif