Key Generation Fixes

[russjones]

Purpose

This PR resolves several issues. The fix for #1640 is uninteresting and #1638 no longer exists due to refactoring in #1679.

#1639 is the core of this PR. The native key generator has been updated to no longer be a singleton, the key pool size has increased, and the pool is used in the recording proxy (which leads to better performance when connecting to a proxy).

Implementation

• Removed the singleton implementation of the native key generator. Instead a single key generator is created per Teleport process that is shared across the service.
• The precomputed keys are now used in the recording proxy, this should decrease time to connect to a node the first time when using the recording proxy.
• Due to the above, the size of the precomputed keys has increased from 10 to 25.

Related Issues

Fixes #1640
Fixes #1639
Fixes #1638

[klizhentas]

I think keygen service should be called on service shutdown in lib/service/service.go then