elasticapm: StartSpan always returns non-nil

[axw]

Transaction.StartSpan and elasticapm.StartSpan now
always return non-nil, even if the transaction is nil.
This simplifies callsites so they don't have to nil
checking of spans.

[codecov-io]

Codecov Report

Merging #92 into master will increase coverage by <.01%.
The diff coverage is 89.13%.

@@            Coverage Diff             @@
##           master      #92      +/-   ##
==========================================
+ Coverage   74.31%   74.32%   +<.01%     
==========================================
  Files          60       60              
  Lines        3134     3131       -3     
==========================================
- Hits         2329     2327       -2     
  Misses        620      620              
+ Partials      185      184       -1

Impacted Files	Coverage Δ
module/apmsql/driver_go110.go	64% <100%> (-1.39%)	⬇️
module/apmsql/stmt.go	58.13% <100%> (-1.87%)	⬇️
module/apmsql/conn.go	55.43% <100%> (-1.86%)	⬇️
module/apmgrpc/client.go	86.66% <100%> (-0.84%)	⬇️
gocontext.go	53.84% <100%> (+7.69%)	⬆️
transaction.go	87.82% <84.84%> (-0.26%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b7c3b32...37f7da5. Read the comment docs.

[axw]

Note that this is not all roses. With this, non-sampled transactions will now incur a bit more overhead due to the allocations. We could return a singleton for dropped spans, but then we could have multiple goroutines racing on updates to the span object. The overhead is kept to a minimum by using a sync.Pool, so I think this is a reasonable trade-off for this safer and neater API.