plugins: EnvGen and Linen check for prehistoric release message
class library: node proxy: avoid hanging notes

When setting the source of a NodeProxy repeatedly and very quickly, the gate = 0 would arrive before the synth was actually running. Now this can be avoided by sending a release time (< -1.0). This fixes #1246.
class library: env release trick not needed anymore.

This is the only potentially intrusive fix in this branch - EnvGate could, since it had the Impulse.ar(0) added to the gate input, be released with gate = 0 even in the same block the synth was created. This was added a while ago to avoid hanging synths in node proxy. Now it needs to be released with a gate <= -1 in the first block, and the node proxy does this.

On the other hand, it can now be started with gate = 0 and started later.
plugins: no change was needed for EnvGen, only Linen failed.

This commit undoes the call of check_gate in EnvGen_Ctor, as it is called in next_k anyway.
Merge pull request #1255 from supercollider/topic/prehistoric-env-release

plugins: Linen checks for prehistoric release message