Fix a couple bugs

[JJL772]

These were issues reported by clang-tidy as I was running that on epics-base.

• AnyScalar::bufferUnsafe() was returning a pointer to the data of an std::string living on the stack.
• ostringstream::str() returns std::string by value, so we can't keep a pointer to its data without first turning it into an lvalue.
• split should take the string as a reference to avoid unnecessary copies.
• Added noexcept to the move ctor/assignment of AnyScalar

[AppVeyorBot]

:white_check_mark: Build pvDataCPP 1.0.35 completed (commit https://github.com/epics-base/pvDataCPP/commit/c3fa20fd53 by @JJL772)