4.x: Improve the performance of ToObservable()

[akarnokd]

This PR improves the performance and allocation behavior of the ToObservable(IEnumerable) operator by inlining the recursive scheduling management and splits the operator into separate, dedicated long-running and recursive variants.

The recursive variant is still declared to not link up the disposable management so the volatile bool _disposed field is enough to signal cancellation. In addition, the previous Action<Action<>>-based recursive scheduling was mostly responsible for the overhead as it was still tracking each recursively scheduled task despite they would have never gotten disposed.

Comparison:

BenchmarkDotNet=v0.10.14, OS=Windows 10.0.17134
Intel Core i7-4790 CPU 3.60GHz (Haswell), 1 CPU, 8 logical and 4 physical cores
Frequency=3513586 Hz, Resolution=284.6095 ns, Timer=TSC
  [Host]     : .NET Framework 4.6.2 (CLR 4.0.30319.42000), 64bit RyuJIT-v4.7.3110.0
  DefaultJob : .NET Framework 4.6.2 (CLR 4.0.30319.42000), 64bit RyuJIT-v4.7.3110.0

// ============================================
// Before

 Method |       N |           Mean |         Error |        StdDev |      Gen 0 |   Allocated |
------- |-------- |---------------:|--------------:|--------------:|-----------:|------------:|
  Exact |       1 |       1.136 us |     0.0122 us |     0.0108 us |     0.2346 |       992 B |
  Exact |      10 |       4.897 us |     0.0435 us |     0.0407 us |     0.5493 |      2312 B |
  Exact |     100 |      43.431 us |     0.5890 us |     0.5221 us |     3.6621 |     15552 B |
  Exact |    1000 |     432.782 us |     3.5242 us |     2.9429 us |    34.6680 |    146291 B |
  Exact |   10000 |   4,298.397 us |    72.5810 us |    67.8924 us |   343.7500 |   1450440 B |
  Exact |  100000 |  41,972.884 us |   240.8476 us |   225.2889 us |  3437.5000 |  14500174 B |
  Exact | 1000000 | 426,720.117 us | 7,334.0969 us | 6,860.3181 us | 34562.5000 | 144987362 B |

// ============================================
// After

 Method |       N |             Mean |          Error |         StdDev |      Gen 0 |  Allocated |
------- |-------- |-----------------:|---------------:|---------------:|-----------:|-----------:|
  Exact |       1 |         738.3 ns |       5.741 ns |       5.370 ns |     0.1497 |      632 B |
  Exact |      10 |       2,464.1 ns |      34.347 ns |      32.128 ns |     0.2861 |     1208 B |
  Exact |     100 |      26,969.5 ns |     167.985 ns |     157.133 ns |     1.6479 |     6968 B |
  Exact |    1000 |     189,754.5 ns |   2,804.522 ns |   2,486.136 ns |    15.3809 |    64569 B |
  Exact |   10000 |   1,882,753.7 ns |   6,135.018 ns |   5,738.699 ns |   152.3438 |   640589 B |
  Exact |  100000 |  26,518,449.9 ns | 180,846.221 ns | 169,163.651 ns |  1500.0000 |  6400644 B |
  Exact | 1000000 | 262,672,144.8 ns | 937,389.291 ns | 830,971.109 ns | 15250.0000 | 64001342 B |