add osmo_stats_report_lock api

Allow multi-threaded access to reported stats:
- enable use of a stats mutex with osmo_stats_report_use_lock(true).
- lock/unlock externally with osmo_stats_report_lock(true/false).

Rationale:

In osmo-hnbgw, we would like to collect stats from nftables, and do so
in a separate thead. The most efficient way is to write the parsing
results from nft directly to the rate_ctr destination.

But what if the main thread reports rate counters at exactly that time?
- is writing to stats atomic on a data type level?
- do applications need stats to be "atomic" as a whole?

In osmo-hnbgw in particular, there are two counters, 'packets' and
'total_bytes'. These correspond, and it would skew statistics if we
reported them out of sync to each other.

The simplest way to ensure correctness in all cases is a mutex around
the stats reporting.

But this mutex isn't needed in most of our programs. To completely avoid
any overhead the mutex may bring, make use of it optional with a global
flag.

This use case is likely to also show up in other programs that would
like to collect and report stats from a separate thread.

Related: SYS#6773
Related: osmo-hnbgw I9dc54e6bc94c553f45adfa71ae8ad70be4afbc8f
Change-Id: Ib335bea7d2a440ca284e6c439066f96456bf2c2d
---
M include/osmocom/core/stats.h
M src/core/libosmocore.map
M src/core/stats.c
M src/vty/stats_vty.c
4 files changed, 128 insertions(+), 0 deletions(-)