init: Initialize globals with kernel::Context's life

...instead of explicitly calling init::{Set,Unset}Globals.

Cool thing about this is that in both the testing and bitcoin-chainstate
codepaths, we no longer need to explicitly unset globals. The
kernel::Context goes out of scope and the globals are unset
"automatically".

Also construct kernel::Context outside of AppInitSanityChecks()

src/kernel/context.h

@@ -5,6 +5,10 @@
 #ifndef BITCOIN_KERNEL_CONTEXT_H
 #define BITCOIN_KERNEL_CONTEXT_H
 
+#include <memory>
+
+class ECCVerifyHandle;
+
 namespace kernel {
 //! Context struct holding the kernel library's logically global state, and
 //! passed to external libbitcoin_kernel functions which need access to this
@@ -14,6 +18,13 @@ namespace kernel {
 //! State stored directly in this struct should be simple. More complex state
 //! should be stored to std::unique_ptr members pointing to opaque types.
 struct Context {
+    std::unique_ptr<ECCVerifyHandle> ecc_verify_handle;
+
+    //! Declare default constructor and destructor that are not inline, so code
+    //! instantiating the kernel::Context struct doesn't need to #include class
+    //! definitions for all the unique_ptr members.
+    Context();
+    ~Context();
 };
 } // namespace kernel