Consolidate the connection pool code at one location

[saurabh500]

The code for SqlClient and Odbc have duplicated the connection pool code.
This code should be consolidated in the Common folder and a single code base should exist for Connection Pool, the same way .Net framework uses it.
This reduces maintenance cost and code duplication as observed in PR dotnet/corefx#26065

[divega]

Now that development of SqlClient has moved to its own repo there cannot be source code reuse, but I believe we can keep this issue here since there is still an opportunity for other ways of code reuse across providers in this or other repos.

[ajcvickers]

Covered by #24856.