### New `dbServerStats()` API for iocStats

A new routine provides the ability to request channel and client counts from
named server layers that implement the `stats()` method, or to get a summary
of the counts from all registered server layers. A preprocessor macro
`HAS_DBSERVER_STATS` macro is defined in the `dbServer.h` header file to
simplify code that needs to support older versions of Base as well.