This is mostly a simple lexical search+replace but the absence of operator< for
std::weak_ptr<T> leads to some complications, particularly with Evoral::Sequence
and ExportPortChannel.
This resolves a PBD vs ARDOUR namespace error for some compilers:
```
error: reference to 'microseconds_t' is ambiguous
libs/pbd/pbd/microseconds.h:29:19: error: candidates are: typedef uint64_t PBD::microseconds_t
libs/ardour/ardour/types.h:81:29: error: typedef PBD::microseconds_t ARDOUR::microseconds_t
```
