Refactor locking; add more debug locking

[tony-iqlusion]

This is an attempt to help address #37.

Based on strace logging it appears at least one of the instances of this bug occurred during a lock acquisition happening immediately after persisting the chain state. The system call sequence looked something like this:

close(12)   = 0
rmdir("/.../.atomicwrite.InysUcmuRax7") = 0
futex(0x..., FUTEX_WAIT_PRIVATE, 2, NULL

Unfortunately this isn't a whole lot to go on, but makes it appear as if it's hanging trying to acquire a lock immediately after persisting the consensus state to disk.

This commit does a couple things to try to narrow down what is happening:

1. Ensures that an exclusive lock to the chain state isn't held while the signing operation is being performed (i.e. while communicating with the HSM). If we were able to update the consensus state, that means the signing operation is authorized, and we no longer need to hold the lock. In the event the signing operation fails, the validator will miss the block in question, but with no risk of double-signing.
2. Adds a significant amount of additional debug logging, particularly around things like lock acquisition and writing to disk. While this commit is unlikely to fix tmkms freeze #37 in and of itself, the additional debug logging should be helpful in isolating the problem.

[codecov-commenter]

Codecov Report

Merging #60 into develop will decrease coverage by 0.36%.
The diff coverage is 1.63%.

@@             Coverage Diff             @@
##           develop      #60      +/-   ##
===========================================
- Coverage    28.85%   28.48%   -0.37%     
===========================================
  Files           50       50              
  Lines         1837     1864      +27     
===========================================
+ Hits           530      531       +1     
- Misses        1307     1333      +26     

Impacted Files	Coverage Δ
src/session.rs	0.00% <0.00%> (ø)
src/chain/state.rs	41.22% <20.00%> (-0.98%)	⬇️

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