BatchSpanProcessor to trigger first export after the interval instead of right away

[cijothomas]

Part of #1968
Similar to https://github.com/open-telemetry/opentelemetry-rust/pull/1766/files, this makes similar change to BatchSpanProcessor.

Before the PR:
Exports were triggered at T, T+60, T+120 and so on, where T is the time BSP was built.

With this change:
Exports are triggered at T+60, T+120 and so on, where T is the time BSP was built.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.9%. Comparing base (4d31408) to head (94d2002).

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #1971     +/-   ##
=======================================
- Coverage   74.9%   74.9%   -0.1%     
=======================================
  Files        122     122             
  Lines      20403   20404      +1     
=======================================
- Hits       15290   15288      -2     
- Misses      5113    5116      +3     

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