Work in progress (w/ API changes)

[mdavidsaver]

I'm creating this PR mainly as a venue to mention several (I think minor) API changes in the big set of changes driven by my porting P4P to use PVXS (including the gateway component). It will likely remain open until I'm closer to finalizing that project.

So far these are:

• Several argument types change from Value&& to const Value&. This should not be a breaking change as an implicit cast is possible. It may trigger warnings about now unnecessary std::move(). This includes: SharedPV::post() and friends, and several of the client *Builder methods.

• The signature of the SharedPV callbacks onFirstConnect() and onLastDisconnect() are changed to add a new argument SharedPV&. This is added for consistency with the onPut() and onRPC() callbacks which already had this convience.

• The server::Config and client::Config classes add a new static method fromEnv() which is identical to the from_env() method. The original from_env() will later be removed.

• Some exception types have changed. I've tried to maintain consistency, but likely haven't been 100% successful.

• Setting an invalid hostname in $EPICS_PVAS_INTF_ADDR_LIST changes from a warning to an error. It didn't seem friendly to silently bind to 0.0.0.0 when a user is explicitly asking for something else.

[AppVeyorBot]

:x: Build pvxs 1.0.279 failed (commit https://github.com/mdavidsaver/pvxs/commit/e0477fbc36 by @mdavidsaver)

[AppVeyorBot]

:white_check_mark: Build pvxs 1.0.288 completed (commit https://github.com/mdavidsaver/pvxs/commit/730f0ddcad by @mdavidsaver)