Added pooling for ServiceRequestBase

[SebastianWe]

After using the fix #428 to make the ExchangeService thread safe, a developer can run into a "ServiceRequestException: The request failed. Connection is still allocated".

This can be fixed by using the ExchangeServiceBase#httpPoolingClient for requests.
This Pull-Requests provides a simple differentiation between the ExchangeServiceBase#httpClient and ExchangeServiceBase#httpPoolingClient.

[azurecla]

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[codecov-io]

Current coverage is 10.42%

Merging #491 into master will not affect coverage as of e6ea8ea

@@            master    #491   diff @@
======================================
  Files          550     550       
  Stmts        20432   20435     +3
  Branches      2625    2626     +1
  Methods          0       0       
======================================
  Hit           2131    2131       
  Partial        153     153       
- Missed       18148   18151     +3

Review entire Coverage Diff as of e6ea8ea

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[serious6]

@SebastianWe thanks for your contribution