[Perf] Inspector refactoring and optimization

[lovromazgon]

Description

The inspector was allocating quite a lot of data even if no sessions were open. Now it's optimized for the hot path when there are no open sessions.

goos: darwin
goarch: amd64
pkg: github.com/conduitio/conduit/pkg/inspector
cpu: Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
                                │  bench.old  │              bench.new               │
                                │   sec/op    │    sec/op     vs base                │
Inspector_NoSession_Send-16       31.96n ± 2%    14.70n ± 1%  -54.01% (p=0.000 n=10)
Inspector_SingleSession_Send-16   92.88n ± 1%   122.15n ± 2%  +31.52% (p=0.000 n=10)
Inspector_10Sessions_Send-16      447.6n ± 2%    447.1n ± 2%        ~ (p=0.895 n=10)
geomean                           109.9n         92.94n       -15.46%

Note that the single session benchmark is slower now because of the use of Record.Clone(), although that usage is now correct, since we previously did not clone all fields. Note that cloning of records was sped up in #1247.

Quick checks:

• I have followed the Code Guidelines.
• There is no other pull request for the same update/change.
• I have written unit tests. (existing tests)
• I have made sure that the PR is of reasonable size and can be easily reviewed.