Add symserv progress and harden against segfaults

[davidanthoff]

This fixes another problem from crash reporting. Sometimes the symbol server crashes with a segfault because a package that we load has some really buggy code. Our try catch for these situations doesn't help. In general, we don't want to send a crash report for those cases: it is not our fault, so we shouldn't count this as a failure on our end.

This PR (and the companion PRs in LS and the client) works around this by adding a back communication channel from the symbol server to the LS process. The symbol server there notifies the LS "I will try to load package X now". If the symserver then crashes, the LS knows that it was a segfault in a package and sends a special trace event via telemetry. As soon as the symserver is done loading the user package it sends a message "I'm done loading package X" to the LS, so the LS now knows that if the symserver crashes after that notification, there is just a bug in the symserver that we should report via crash reporting.

As a small side bonus we get nicer progress reporting for the indexing process :)

Requires https://github.com/julia-vscode/LanguageServer.jl/pull/591 and https://github.com/julia-vscode/julia-vscode/pull/1043.

This is finicky to test, but I did try out a whole variety of different crash situations and it seems to work.

[codecov[bot]]

Codecov Report

Merging #114 into master will decrease coverage by 4.96%. The diff coverage is 82.35%.

@@            Coverage Diff            @@
##           master    #114      +/-   ##
=========================================
- Coverage   80.16%   75.2%   -4.97%     
=========================================
  Files           5       5              
  Lines         595     621      +26     
=========================================
- Hits          477     467      -10     
- Misses        118     154      +36

Impacted Files	Coverage Δ
src/server.jl	80% <ø> (-8.89%)	:arrow_down:
src/symbols.jl	84.5% <100%> (-0.39%)	:arrow_down:
src/SymbolServer.jl	84.14% <80%> (-1.31%)	:arrow_down:
src/utils.jl	33.62% <0%> (-26.55%)	:arrow_down:

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