Amplitude client using ForkJoinPool for I/O

[kevink-sq]

Expected Behavior

sendEvents should use sendThreadPool instead of the default ForkJoinPool.

Current Behavior

sendEvents uses the default ForkJoinPool via CompletableFuture.supplyAsync(runnable) which scales with the number of cpu.

Possible Solution

See #107

[kevink-sq]

cc: @izaaz