Switch to System.Text.Json

[mfkl]

Rebased and iterated on #73

Few points to discuss:

• Bundled a few needed ConfigurationAwait(false) in the initial commit.
• which version of Text.Json do we want? This comment is on point but over a year passed, so we can probably pick a reasonable version now.
• Marcin dynamic comment,
• do we want to keep a netcoreapp2.1 target? f042deb

Closes #43

[mfkl]

netcoreapp2.1 failures are expected but there is a net461 timing one.

[xUnit.net 00:01:01.68]     Consul.Test.LockTest.Lock_ContendFast [FAIL]
Error: Consul.Test.LockTest.Lock_ContendFast: Consul.ConsulRequestException : Unexpected response, status code InternalServerError: invalid session "3103bd01-8165-4b14-cddd-542796ae036a"
  Failed Consul.Test.LockTest.Lock_ContendFast [31 s]
  Error Message:
   Consul.ConsulRequestException : Unexpected response, status code InternalServerError: invalid session "3103bd01-8165-4b14-cddd-542796ae036a"
  Stack Trace:
    at Consul.PutRequest`2[TIn,TOut].Execute (System.Threading.CancellationToken ct) [0x002c5] in <04779d9f65704d909393c78b5c007b81>:0 
  at Consul.Lock.Acquire (System.Threading.CancellationToken ct) [0x00447] in <04779d9f65704d909393c78b5c007b81>:0 
  at Consul.Lock.Acquire (System.Threading.CancellationToken ct) [0x00797] in <04779d9f65704d909393c78b5c007b81>:0 
  at Consul.Test.LockTest+<>c__DisplayClass9_1.<Lock_ContendFast>b__0 () [0x00096] in <3c10[17](https://github.com/G-Research/consuldotnet/runs/7343250916?check_suite_focus=true#step:9:18)3e055c4e98a841ccf76066[21](https://github.com/G-Research/consuldotnet/runs/7343250916?check_suite_focus=true#step:9:22)5d>:0 
  at Consul.Test.TimeoutUtils.WithTimeout (System.Threading.Tasks.Task task) [0x000fe] in <3c10173e055c4e98a841ccf76066215d>:0 
  at Consul.Test.LockTest.Lock_ContendFast () [0x000d8] in <3c10173e055c4e98a841ccf76066215d>:0

[mfkl]

If we want to bump the minor version to 1.7 for this change, we may want to provide bindings for all 1.7 Consul APIs, such as Namespaces #117. This needs a consul enterprise version, so I reached out to Consul for it. Let's see.

[winstxnhdw]

We can probably continue with this.

[marcin-krystianc]

We can probably continue with this.

@winstxnhdw Feel free to give it a try.